Check BaseUser.class.php in /activecollab/application/modules/system/models/users to see how activeCollab objects connect to database tables and how fields are mapped.
There is no such thing as user tagging, but it can be added the way you described it (by adding a new field).
How can we add new fields for users? I need to add fields such as personal email address, skills set etc. for every user.
Also, is there a tag system for users such that we can add tags based on skill sets for every user?
Thanks,
Deep