Pre-Meeting Breakout Rooms

Hi all,

I am trying to add a group (90 people max) to different breakout rooms. I do this before the meeting starts in the breakout rooms tab. However, when I want to add participants to certain rooms i cannot find them all back in my participants list. I did invite everyone, is there an explanation for this? I can add some e-mail addresses that ends with hotmail or gmail.com to a breakout room but not all of them. It takes quite some time to have to manually add everyone to breakout rooms during the meeting.