Is there a CONF file for groups???

[pclapham at windstream.net]

Hi --
I have set up a server with Karmic and about 30 users, each of which belongs to group "students." This group is their primary and only group. The problem is that they cannot log into the system! Not locally, not via ssh, not via samba. It would appear that new groups do not have, by default, logon privilege. Does anybody know of a conf file that would give specific privileges (such as access to the system) to a group?
Thanks.
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