This case, like its predecessor, Cost Mgmt. Group v. Bommer, 327 Ga. App. 164 (755 SE2d 537) (2014) (hereinafter CMG I), involves competing lawsuits between former business associates over disputes that include contract issues and restrictive covenants. The first suit was litigated in federal court.1 The other suit remains pending in the Superior Court of Fulton County. In the federal suit, Daniel Bommer sued his former business partners, Steven Gareleck and George Remington Reynolds, in their individual capacities. In the superior court action, companies controlled by Gareleck and Rey*399nolds — Cost Management Group, Inc., Partnertel, Inc., and Telcentrex, LLC (collectively “CMG”) — sued Bommer.
The latest iteration of this case is before us on appeal from the superior court’s sua sponte grant of summary judgment to Bommer. CMG argues in three enumerations that the trial court erred in finding that its amended claims in the superior court suit were barred as unasserted compulsory counterclaims in the prior federal action. For the reasons that follow, we reverse.
Summary judgment is proper when there is no genuine issue of material fact and the movant is entitled to judgment as a matter of law. A de novo standard of review applies to an appeal from a grant of summary judgment, and we view the evidence, and all reasonable conclusions and inferences drawn from it, in the light most favorable to the nonmovant.
(Footnote omitted.) Coffee Iron Works v. QORE, Inc., 322 Ga. App. 137, 137 (744 SE2d 114) (2013). See generally McClendon v. 1152 Spring St. Assocs.-Ga., Ltd. III, 225 Ga. App. 333, 334 (484 SE2d 40) (1997) (a trial court may sua sponte grant summary judgment on issues not raised by parties).
The facts as outlined in our earlier opinion also apply to the instant case, and we adopt them here as necessary. See CMG I. In CMG I, we reversed the trial court’s grant of partial summary judgment to Bommer and determined that CMG was not collaterally estopped by the federal action from pursuing, in the superior court action, claims relating to Bommer’s operation of a company called Enhanced Billing Services (EBS). Id. at 168 (1). We also determined that the claims asserted in superior court that related to EBS were not compulsory counterclaims in the federal action. Id. at 172 (2).
However, in that grant of partial summary judgment to Bommer, the trial court also ordered CMG to file a brief addressing why “this entire case should not be dismissed with prejudice.” Following the parties’ submission of briefs, the trial court entered a final order granting summary judgment to Bommer on CMG’s remaining claims that were unrelated to Bommer’s operation of EBS, including, inter alia, claims for breach of fiduciary duty, fraud, and breach of restrictive covenants. CMG appeals that ruling in the instant action.
1. CMG argues that the trial court erred in finding that its claims that are unrelated to Bommer’s operation of EBS are barred in the superior court action as unasserted compulsory counterclaims. Specifically, CMG contends that its claims are not barred because, as a nonparty to the federal action, it does not qualify as a “pleader” under federal law for purposes of establishing a compulsory counterclaim. *400We agree.2
As we noted in CMG I, the federal court in the earlier action in which Bommer sued Gareleck and Reynolds exercised diversity jurisdiction. See Bommer v. Reynolds, 465 Fed. Appx. 876 (11th Cir. 2012). A federal court sitting in diversity must apply state substantive law and federal procedural law. Esfeld v. Costa Crociere, S. P. A., 289 F3d 1300, 1306 (II) (A) (11th Cir. 2002). A state court’s determination of whether an issue is substantive or procedural, for choice-of-law purposes, is binding on a federal court. Shaps v. Provident Life & Accident Ins. Co., 244 F3d 876, 883 (II) (11th Cir. 2001). This Court has determined that “OCGA § 9-11-13 (a), which governs compulsory counterclaims in Georgia, sets forth the method for bringing such claim[s] and thus is procedural in nature.” Harper v. Harper, 267 Ga. App. 553, 555 (1) (600 SE2d 659) (2004). As a result, in determining whether the claims at issue here were compulsory counterclaims, we must look to federal procedural law.
Fed. R. Civ. P. 13 (a) (1) provides as to compulsory counterclaims that
[a] pleading must state as a counterclaim any claim that — at the time of its service — the pleader has against an opposing party if the claim: (A) arises out of the transaction or occurrence that is the subject matter of the opposing party’s claim-, and (B) does not require adding another party over whom the court cannot acquire jurisdiction.
(Emphasis supplied.)
It is undisputed that CMG was not a party to the federal action. See generally CMG I, supra at 172 (2). Further, the trial court’s opinion acknowledged this in its discussion of Montgomery Ward Dev. Corp. v. Juster, 932 F2d 1378 (11th Cir. 1991). In Montgomery Ward, the Eleventh Circuit determined that
a potential defendant who was not joined in the prior action... does not qualify as a pleader for purposes of the compulsory counterclaim bar. We do not find that appellant. .. who .. . was not joined as a defendant in the [initial] suit, can be *401barred for the very reason that [plaintiff-appellee] did not join him. Since [appellant] did not join in any pleading and had no obligation to join the [initial] suit, he cannot be barred now by a failure to have done so.
Id. at 1383 (B). The trial court wrote that although CMG “correctly cite[s] Montgomery, that case is distinguishable from the case at bar” because the nonparty in Montgomery
had no apparent connection to, or authority to act for, the company who had been the original defendant. . . . Here, unlike in Montgomery, Gareleck and Reynolds have a very direct connection with, and authority to act on behalf of [CMG] ... as 95% shareholders, directors, and officers of CMG.3
The trial court cites no law supporting its determination that, as a nonparty to the federal suit, CMG qualified as a “pleader” under Fed. R. Civ. P. 13 (a) on the basis that the actual parties — Gareleck and Reynolds — had a “direct connection” with and “authority to act” on behalf of CMG. We find no law from this jurisdiction indicating that either a “direct connection” with or “authority to act” on behalf of a party turns a nonparty into a pleader under Fed. R. Civ. P. 13 (a).4 Nor did CMG have any obligation to intervene in the federal suit. See Mann v. City of Albany, Ga., 883 F2d 999, 1005 (III), n. 2 (11th Cir. 1989); Barham v. City of Atlanta, 292 Ga. 375, 377 (1) (b) (738 SE2d 52) (2013). As a result, we find Montgomery Ward, where our Eleventh Circuit determined that a successor-in-interest pendente lite who was not a party to the prior suit did not qualify as a pleader for purposes of the compulsory counterclaim bar, to be controlling in the instant action.5 Montgomery Ward, supra at 1383 (B).
*402Decided May 30, 2014
Raley & Sandifer, G. Brian Raley, Michael A. Sierra, for appellants.
Thompson Hine, Gary S. Freed, Fisher Broyles, Milo S. Cogan, for appellee.
2. As a result of our determination in Division 1, we need not reach CMG’s remaining enumerations of error.
Judgment reversed.
Barnes, P. J., and Miller, J., concur.