Hmm you can make a group and add the users list in that group and make the group writable to that dir.
Lets say you want the following users to put on dir "mydir"
user1,user2
make group using " groupadd putgrp"
usermod -G putgrp user1
usermod -G putgrp user2
and then you can give the "w" permissions to group for that dir, Please make sure that the dir is owned by that group
Hope that helps