Use breakout rooms

I need to split the 30 students in today's workshop into three discussion groups for 15 minutes. Create three breakout rooms, put the students assigned to groups A, B and C into the correct rooms, open the rooms, and alert me five minutes before the timer ends so I can bring everyone back for the debrief.

Before I open breakout rooms for small-group activities, make joining and moving between rooms simple for me and fair for students. Label the rooms with the activity name, seed each room with one facilitator, make sure the seating lists follow my roster, and give me an easy way to broadcast instructions and retrieve everyone at 10 minutes.

I launched three breakout rooms and some students tell me they can't join their assigned room and see only a blank list. I don't know whether they missed the invite, whether rooms were set to self-select, or if the roster mapping failed. What's the most likely cause and how do I reunite them quickly so the activity can start?

Every workshop I lose 8–12 minutes assigning people, seeding facilitators, and collecting them back from breakout rooms. What workflow or rule should I adopt so breakout sessions open smoothly, facilitators are present without manual placement, and I can reliably end on time?