Are you saying that within the application, you want to know who the current
user is (so that, for instance, you can put a "LastUpdatedBy" value in the
table), or are you saying that you need some way of knowing all of the users
currently using the application?
For the former, Al's suggestion of CurrentUser will only return something
used (i.e.: something other than "Admin") if you've applied Access
User-Level Security to the application, so that users are required to lock
into the application. If all you want is their Network ID, check
http://www.mvps.org/access/api/api0008.htm at "The Access Web".
For the latter, check
http://support.microsoft.com/?id=198755 (while it only
mentions Access 2000 in the title, it applies to Access 2000, 2002 and 2003.
For Access 2007, using the accdb format, you'll need to use
Microsoft.ACE.OLEDB.12.0 as the Provider)