assign role to group

Hi All,

I have a lot of general users, in one unix group, and I would like to assign
a custom role for all users. It would be very easy, if I could assign a role
for that group - but as I see, I can not do this. Do you have any idea, or
recommendation, how can I accomplish this? I wouldn't assign the role in
every time one by one, when add a new user.
 Unfortunately, they have to log in, and have a shell, because they use a
terminal based local application via telnet/ssh.

Thanks for your help,
Tamas