Unique users in Ofuser table

We have 4 websites where users are going to mapped to one single instance of Openfire for one to one chat. We have created 4 groups however unique userID of each user on every site cannot be assured. Is there any way we can have one more primary key along with UserID so that duplicate user in different group i.e different website do not make any clash … Can we have two columns in Ofuser for checking the uniqueness.

Please advice how can we handle such situation.

Rgds,