AgoraVideoViewController.swift

var remoteUserIDs: [UInt] = []

...

extension AgoraVideoViewController: AgoraRtcEngineDelegate {
    func rtcEngine(_ engine: AgoraRtcEngineKit, didJoinChannel channel: String, withUid uid: UInt, elapsed: Int) {
        callID = uid
    }

    func rtcEngine(_ engine: AgoraRtcEngineKit, didJoinedOfUid uid: UInt, elapsed: Int) {
        print("Joined call of uid: \(uid)")
        remoteUserIDs.append(uid)
        collectionView.reloadData()
    }

    func rtcEngine(_ engine: AgoraRtcEngineKit, didOfflineOfUid uid: UInt, reason: AgoraUserOfflineReason) {
        if let index = remoteUserIDs.firstIndex(where: { $0 == uid }) {
            remoteUserIDs.remove(at: index)
            collectionView.reloadData()
        }
    }
}