diff --git a/app/features/tournament/actions/to.$id.register.server.test.ts b/app/features/tournament/actions/to.$id.register.server.test.ts
new file mode 100644
index 000000000..5e0ebca3b
--- /dev/null
+++ b/app/features/tournament/actions/to.$id.register.server.test.ts
@@ -0,0 +1,117 @@
+import { addDays } from "date-fns";
+import { describe, expect, test, vi } from "vitest";
+import * as FriendshipFactory from "~/db/seed/factories/FriendshipFactory";
+import * as TournamentFactory from "~/db/seed/factories/TournamentFactory";
+import * as TournamentTeamFactory from "~/db/seed/factories/TournamentTeamFactory";
+import * as UserFactory from "~/db/seed/factories/UserFactory";
+import { db } from "~/db/sql";
+import { dateToDatabaseTimestamp } from "~/utils/dates";
+import { assertResponseErrored, wrappedAction } from "~/utils/Test";
+import type { registerSchema } from "../tournament-schemas.server";
+import { action as registerAction } from "./to.$id.register.server";
+
+// adding a player notifies them; not under test here
+vi.mock("~/features/notifications/core/notify.server", () => ({
+	notify: () => Promise.resolve(),
+}));
+
+/** `maxMembersPerTeam` of a regular 4v4 tournament that sets no limit of its own. */
+const MAX_ROSTER_SIZE = 6;
+
+const register = wrappedAction<ReturnType<typeof registerSchema>>({
+	action: registerAction,
+	isJsonSubmission: true,
+});
+
+const memberCountOfOnlyTeam = async (tournamentId: number) => {
+	const { count } = await db
+		.selectFrom("TournamentTeamMember")
+		.innerJoin(
+			"TournamentTeam",
+			"TournamentTeam.id",
+			"TournamentTeamMember.tournamentTeamId",
+		)
+		.select((eb) => eb.fn.countAll<number>().as("count"))
+		.where("TournamentTeam.tournamentId", "=", tournamentId)
+		.executeTakeFirstOrThrow();
+
+	return count;
+};
+
+/**
+ * A tournament open for registration with one team of `rosterSize` captained by the
+ * user the action submits as, and a friend of theirs available to be added to it.
+ */
+const scenario = async ({
+	rosterSize,
+	requireInGameNames = false,
+}: {
+	rosterSize: number;
+	requireInGameNames?: boolean;
+}) => {
+	// the captain, who submits the request
+	const captain = await UserFactory.createRegular({
+		profile: requireInGameNames ? { inGameName: "Captain#1234" } : null,
+	});
+	// the player they try to add, without an in-game name of their own
+	const friend = await UserFactory.create({ profile: null });
+	const teammates = await UserFactory.createMany(rosterSize - 1);
+
+	await FriendshipFactory.create({
+		userOneId: captain.id,
+		userTwoId: friend.id,
+	});
+
+	const tournament = await TournamentFactory.create({
+		authorId: captain.id,
+		startTimes: [dateToDatabaseTimestamp(addDays(new Date(), 7))],
+		requireInGameNames,
+	});
+
+	await TournamentTeamFactory.create({
+		tournamentId: tournament.id,
+		memberUserIds: [captain.id, ...teammates.map((user) => user.id)],
+	});
+
+	return {
+		tournamentId: tournament.id,
+		addFriend: () =>
+			register(
+				{ _action: "ADD_PLAYER", userId: friend.id },
+				{ user: "regular", params: { id: String(tournament.id) } },
+			),
+	};
+};
+
+describe("Tournament registration ADD_PLAYER", () => {
+	test("adds a player to a team with room left", async () => {
+		const { tournamentId, addFriend } = await scenario({
+			rosterSize: MAX_ROSTER_SIZE - 1,
+		});
+
+		await addFriend();
+
+		expect(await memberCountOfOnlyTeam(tournamentId)).toBe(MAX_ROSTER_SIZE);
+	});
+
+	test("does not add a player to a team already at max capacity", async () => {
+		const { tournamentId, addFriend } = await scenario({
+			rosterSize: MAX_ROSTER_SIZE,
+		});
+
+		assertResponseErrored(await addFriend(), "Team is already at max capacity");
+
+		expect(await memberCountOfOnlyTeam(tournamentId)).toBe(MAX_ROSTER_SIZE);
+	});
+
+	test("does not add a player without an in-game name when the tournament requires one", async () => {
+		const { tournamentId, addFriend } = await scenario({
+			rosterSize: 1,
+			requireInGameNames: true,
+		});
+
+		assertResponseErrored(await addFriend(), "no in-game name");
+
+		expect(await memberCountOfOnlyTeam(tournamentId)).toBe(1);
+	});
+});
diff --git a/app/features/tournament/actions/to.$id.register.server.ts b/app/features/tournament/actions/to.$id.register.server.ts
index 9cbc308ec..a20c9037b 100644
--- a/app/features/tournament/actions/to.$id.register.server.ts
+++ b/app/features/tournament/actions/to.$id.register.server.ts
@@ -269,16 +269,25 @@ export const action: ActionFunction = async ({ request, params }) => {
 				"User is already in a team",
 			);
 			errorToastIfFalsy(ownTeam, "You are not registered to this tournament");
+			errorToastIfFalsy(
+				ownTeam.memberUserIds.length < tournament.maxMembersPerTeam,
+				"Team is already at max capacity",
+			);
 			errorToastIfFalsy(
 				(await SQGroupRepository.findFriendsAndTeammates(user.id)).friends.some(
 					(friendPlayer) => friendPlayer.id === data.userId,
 				),
 				"Not a friend",
 			);
+			const userToAdd = await UserRepository.findLeanById(data.userId);
 			errorToastIfFalsy(
-				(await UserRepository.findLeanById(data.userId))?.friendCode,
+				userToAdd?.friendCode,
 				"User you are trying to add has no friend code set",
 			);
+			errorToastIfFalsy(
+				!tournament.ctx.settings.requireInGameNames || userToAdd.inGameName,
+				"User you are trying to add has no in-game name set",
+			);
 			errorToastIfFalsy(tournament.registrationOpen, "Registration is closed");
 
 			await requireNotBannedByOrganization({