What is the ruling on gesturing with the hand during prayer with the intention of alerting a congregant to the presence of another congregation, and is it obligatory to interrupt the prayer and resume it with that congregation?

He who finds a congregation should join them in prayer and not pray alone. If he intentionally prays alone, he sins and forfeits the reward of congregational prayer. Gesturing with the hand and eye during prayer is permissible and does not invalidate the prayer, as long as the aforementioned person has followed you (in prayer), then both of you will attain the virtue of congregational prayer.