[jamoss]

For the home page active users, edit index.php

SEARCH FOR (around line 401):
WHERE session.lastactivity > $datecut

AFTER THAT, ADD:
AND user.usergroupid in (x,x,x)

... where x,x,x are the usergroups you want it to show.

For the Member List feature, you can create a plugin:

Hook Location: memberlist_query_userscount
PHP CODE:
$ids='x,x,x';

where x,x,x are the usergroups you want it to show.