How raid ID works?

Hi everyone, how does it work? If, for example, in RS 10 on Halion, a raid breaks up, and 2 people from this raid with the same ID assembling 2 separate groups, and this groups are full, and they start going inside, who will have the advantage of entering? And if they go at the same time, what will happen then? Or maybe you have your variant? Explain please! Thanks!

Instance will let in only 10 people, for example, 4 from group#1 and 6 from group#2. Rest will get "instance is full" message.

Tough, this happened to me years ago, it may work different now.