groups and directories

[Wojciech Puchar]

assume we have timesharing system and multiple users.

everyone have his/her home directory and here - the access right and 
ownership is simple.

assume we need two shared directories - a and b

directory a must be for user1,user2 and user3, directory b for user3,user4 
and user5.

things are simple - i create groups a and b, and add user1,2,3 to group a, 
and user3,4,5 to group b.

when any of them create file in those directories, group is automatically 
added.

setting umask to 007 allows anyone else from group to read and modify 
those files.


but when user1 create file first at home directory and then move it to 
directory a, it still is in group user1.

is it possible to make automatic chgrp anyway? or just completely 
different solution.

I don't mean samba/ftp/nfs but directly accessing files by users working 
on the same machine.