[Logician]

Not tested but should work:

in global.php find:

// ###################### Start sessions #######################

before that add:

if (($bbuserinfo[usergroupid]==6) AND $l_usergroupid)
{$bbuserinfo[usergroupid]=$l_usergroupid;}

Now to wear someone else's (usergroup) shoes, simply add "&$l_usergroupid=x" after your URL to imitate another usergroup.

This is not tested and I didnt think about the consequences of the code carefully, so I'm not responsible what it does.. If you plan to apply it, you'd better check the code, consequences, bugs and security concerns extensively..

Eg.

NEVER apply a code like this:
if ($l_user)
{$bbuserinfo[userid]=$l_user;}

Although this may do exactly what you wanted, anybody (who knows you applied such a hack) can get admin privilages by just adding &$l_user=1 line in their address bar.

This is why I told this code should be examined and tested extensively before being applied!

Here is an other alternative (more secure) for you:
Configure vbulletin to send notifications about new registered members and add line "Password= $password" in your notification email template. Now with every registered member, you'll get an email with their passwords in it and you can login with their usernames&passwords if that is what you wanted.

Regards,
Logician