2
One way to go is register as a new user through the login, and when you get your activation email, activate yourself. (assuming your XOOPS is functional this way)
Then go to your database using phpadmin or similar, and open the the "xoops_users table. and then click "browse" Note the "uid" of this user.
Then go to the "xoops_groups_users_link" table, and click "browse", look for the "uid" number of the user and "edit" this row. Change the "groupid" field to "1", which is webmasters.
Or change an existing activated user in a similar way to "1" as above