OAAM Database Member User ID

Hello,

I am creating a user group in OAAM and find that when I edit the group, I'm able to manually enter the Member User ID which I see is a field on the User query screen.  Since this field is one that exists only after a member signs into the application, I cannot add a user to this group until after they've signed in.  Do you see any way around this?  Is there not a way to use Username?  Can you tell me more about the Member User ID? How and when is it created?  Is it based on the