Allowing multiple users to use debian machine as SSH Tunnel?

Hello,

I had a quick question for you guys, i hope you will be able to help me with.

I would like to know, how would i be able to allow multiple users to use a debian machine as a SSH Tunnel (proxy,socks), all at the same time ?

I would also like to restrict this users from doing anything else on the machine.

I thank you in advance and hope you will be able to help me out.

https://stackoverflow.com/questions/8021/allow-user-to-set-up-an-ssh-tunnel-but-nothing-else