Cant join meeting after it began even tho I am a co-host

[Sumpte10]

I created a meeting for my boss and her associates. I made all of them co-hosts. My boss logged on, I was not. After an hour one of her associates needed to present some info on her laptop/share screen. She tried to log on and join the meeting. She was a co-host. It wouldnt let her join. She went to her zoom and tried to "Join A Meeting" by entering the Meeting ID but it told her she needed to register. What does this mean? She has a ZOOM account thru our organization. Should I have not made her a cohost? I dont understand why she couldnt join? Obviously everyone joins after a meeting has begun, except the first person so why didnt it allow her?