Code to change usergroup at registration

[kirstyd]

I'm wanting to put users who meet certain conditions when they register into the moderated user group so we can check them before letting them in. I've added a plugin at the register_addmember_complete hook which identifies the prospective users just fine, but I can't figure out the right bit of code to get them put into the moderated user group (which is group id 4).

I'm trying:

 $userdata->user[usergroupid]=4;

but it's having no effect.

Can anyone help me out with the correct line of code please?

thanks
kirsty