(1) Nope -- in the traditional Unix sense, a file system belongs to one user, and one group. If user/group access permissions are not sufficient, many filesystems these days support access control lists (ACLs) a la Windows, but it's something that usually has to be specially enabled.
(3) id <user> or groups <user>
(4) The user's primary group is the 4th field in /etc/passwd -- otherwise you can add the user to the appropraite group entry in /etc/group. An /etc/group entry looks like
* group passwords aren't used anymore. They were useful way back in the day when a user could only belong to one group at a time.
to remove a user from a group, just delete his name from the corresponding line. There are pretty GUI tools to do it, too.