Hi Peter!
I am happy to have you here, welcome

Hopefully you stay

so lets start have a look at your question(s).
At first, there should be no problem at all, managing this amount of User.
You said nothing about their memberships, but I guess not all of them will join the same Usergroup?!?
Managing them in a couple (or more) of groups should be quite easy with a good pre-work (Konzept).
Are you going to manage them in the CMS or do you have a LDAP/Wiki/Forum or anything else where the accounts already exist?
BIGACE uses a plugabble Authentication system, which might be customized for special purposes.
Probably we will have to enhance the User Administration Frontend to add some sort of Search functionality (which is already existing in the PHP Backend).
Also it might be an goog idea to split the User Administration into pages, cause 500+ User might be a little hard to scroll through

If you have special fields (like contact information) which are not supported currently, you have to customize only one config file to add those new fields to the Userattributes.
But as long as you have no more concrete question, let me say:
I would really appreciate it to get some active support (even questions are helpful to see what is needed!).
Kevin