I have three users in Redhat 6 machine,tigergouravsouravUser gourav and sourav in brother group.Now user tiger create a directory tiger_gaurav and want to give read and write permission to only and only gourav user.When I try to give permission according to group,sourav also gets the permission to access that directory.Please do help me with this.  
Give full permissions to the owner and only one other user
files;permissions
Using ACL's this can be done (Red Hat - Setting Access ACLs) - $ setfacl -m u:gourav:rwx,d:u:gourav:rwx,d:u:tiger:rwx,m:rwx path/tiger_gauravu:gourav:rwx  grants user gourav read,write, and execute to the directoryd:u:gourav:rwx  sets the default rule that will grant user gourav read,write,execute permission to files created in the directoryd:u:tiger:rwx  sets the default rule that will grant user tiger read,write,execute permission to files created in the directorym:rwx  sets the mask, this value is unioned with the owning group and all other users/groups.That said, creating a group is likely much simpler to maintain in the long run. 