@kaype, if you want to force the same signature for all users in a group, a slightly different method should be used:
PHP Code:
if (is_member_of($this->post, $my_usergroup_here))
{
$this->post['signature'] = 'THIS IS A FORCED USERGROUP SIGGY!';
}
That will assign the signature to all members of a group, regardless of what they put in their signature box.
Forcing it on registration to a certain group is possible, too, but it would require some different methods. It also depends on if you want users to be able to change their signature at a later date. (The method above will not allow them to change their signature while they are a member of that group. Well, more accurately, they can make all the changes they want, but that sig you set will always be what shows up.)