diff --git a/MyPassCore/Sources/MyPassCore/Session/GroupSelection.swift b/MyPassCore/Sources/MyPassCore/Session/GroupSelection.swift
new file mode 100644
index 0000000..6b346e4
--- /dev/null
+++ b/MyPassCore/Sources/MyPassCore/Session/GroupSelection.swift
@@ -0,0 +1,38 @@
+import Foundation
+
+public enum GroupSelection {
+    /// `groupId` and all of its descendant subgroup IDs, found by searching `root`'s tree.
+    /// Returns an empty set if `groupId` isn't found anywhere in `root`.
+    public static func subtreeIds(of groupId: UUID, in root: Group) -> Set<UUID> {
+        guard let found = findGroup(groupId, in: root) else { return [] }
+        return allIds(in: found)
+    }
+
+    /// Toggles `groupId` and its full subtree in `current`: if the group is already selected,
+    /// it and its subtree are removed; otherwise they're added.
+    public static func toggling(_ groupId: UUID, in root: Group, current: Set<UUID>) -> Set<UUID> {
+        let subtree = subtreeIds(of: groupId, in: root)
+        guard !subtree.isEmpty else { return current }
+        if current.contains(groupId) {
+            return current.subtracting(subtree)
+        } else {
+            return current.union(subtree)
+        }
+    }
+
+    private static func findGroup(_ id: UUID, in group: Group) -> Group? {
+        if group.id == id { return group }
+        for sub in group.subgroups {
+            if let found = findGroup(id, in: sub) { return found }
+        }
+        return nil
+    }
+
+    private static func allIds(in group: Group) -> Set<UUID> {
+        var ids: Set<UUID> = [group.id]
+        for sub in group.subgroups {
+            ids.formUnion(allIds(in: sub))
+        }
+        return ids
+    }
+}
diff --git a/MyPassCore/Tests/MyPassCoreTests/GroupSelectionTests.swift b/MyPassCore/Tests/MyPassCoreTests/GroupSelectionTests.swift
new file mode 100644
index 0000000..b5c5a84
--- /dev/null
+++ b/MyPassCore/Tests/MyPassCoreTests/GroupSelectionTests.swift
@@ -0,0 +1,42 @@
+import XCTest
+@testable import MyPassCore
+
+final class GroupSelectionTests: XCTestCase {
+    private func makeSampleTree() -> (root: Group, work: Group, email: Group, personal: Group) {
+        let email = Group(name: "Email")
+        let work = Group(name: "Work", subgroups: [email])
+        let personal = Group(name: "Personal")
+        let root = Group(name: "Root", subgroups: [work, personal])
+        return (root, work, email, personal)
+    }
+
+    func test_subtreeIds_includesGroupAndAllDescendants() {
+        let (root, work, email, _) = makeSampleTree()
+        let ids = GroupSelection.subtreeIds(of: work.id, in: root)
+        XCTAssertEqual(ids, [work.id, email.id])
+    }
+
+    func test_subtreeIds_leafGroup_returnsOnlyItself() {
+        let (root, _, email, _) = makeSampleTree()
+        let ids = GroupSelection.subtreeIds(of: email.id, in: root)
+        XCTAssertEqual(ids, [email.id])
+    }
+
+    func test_toggling_selectsGroupAndSubtree() {
+        let (root, work, email, _) = makeSampleTree()
+        let result = GroupSelection.toggling(work.id, in: root, current: [])
+        XCTAssertEqual(result, [work.id, email.id])
+    }
+
+    func test_toggling_deselectsGroupAndSubtree() {
+        let (root, work, email, _) = makeSampleTree()
+        let result = GroupSelection.toggling(work.id, in: root, current: [work.id, email.id])
+        XCTAssertEqual(result, [])
+    }
+
+    func test_toggling_doesNotAffectUnrelatedSelection() {
+        let (root, work, email, personal) = makeSampleTree()
+        let result = GroupSelection.toggling(work.id, in: root, current: [personal.id])
+        XCTAssertEqual(result, [personal.id, work.id, email.id])
+    }
+}