How can latecomers inform someone performing a supererogatory prayer of their intention to join him in congregational prayer?

If he enters the mosque and wishes to follow someone performing a supererogatory prayer, he may follow him without informing him, because the intention to lead is not a condition for the validity of the following prayer, but rather it is recommended. If he wishes to inform him, he may do so by any permissible method, such as placing a hand on his shoulder or informing him with an audible voice, and there is no specific method prescribed by Islamic law for this.