BSCW lets you search for users -- either in your own Address Book or among all registered users of the server ("the whole system"). The second option can be particularly helpful if you want to invite someone into a Workspace who is a user but whose user name and email address you do not know.
As search criteria you may specify fields of the 'Details' form, which every registered user should have filled in. On many BSCW servers, however, the only information that is likely to be known for each user is 'user name' und 'email address'.
If you have found the entry of the user that you are looking for, in the Search Result page,
or 
on the button bar for selected objects to bring up the respective form for sending her/him an email.
or
on the button bar for selected objects to add the user name (or the email address of a pending user) to your Address Book.