You can create breakout groups by click on the gear and then Create breakout groups:

You can check on the groups by clicking Breakout Rooms and then Join Room.

When you join a room, it opens a second window:

NOTES: 

• Breakout rooms are not included in class recordings.
• Students must be IN the conference to be assigned a breakout room.
• Breakout rooms are one and done.  If a student arrives late, promote them to moderator so they can join a room.
• If you have multiple breakout rooms open, be sure to leave and rejoin the audio

Need to know more?  Check out this documentation from Big Blue Button