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Will you have access to anything more than SQL queries?
This ought to work:
INSERT INTO `xoops_users` (uname, email, pass) VALUES ('$uname', '$email', '$pass')
for creating the user - how you get the username, email and password and get it into the SQL query, I'll let you find out
Now, the next is a little more tricky as you will need to get the insert id of the just executed query (In XOOPS we use a method, which will execute mysql_insert_id($this->conn);)
Then you should use that insert ID in the following:
INSERT INTO `xoops_groups_users_link` (groupid, uid) VALUES (2, $insertID)
To put the user in the registered users group.
of course, if you have access to call a php page from your game script, it could be somewhat easier.