if you don't want to change anything, you receive nothing. there is a way without changing the ownership, but it is not the way you want, because it requires a little bit deeper understanding of AIX permission system and can cause some strange behaviour if it is applied without knowledge.

The way you want to go is to make one other group, say rwgroup. Users, who must have the right to write in the directory, receive this group as their secondary group. You change the owning group of the directory to rwgroup and set permissions to 2775. In this case all the users, who belong to rwgroup, can write to the directory. All the files they create in the directory will be created with rwgroup. All users, who don't belong to rwgroup, has read (list) access to the directory.

In this case you solve your task in a portable and clear for every administrator (and every tool) manner. If you choose another way, say allow 2 groups to write to one directory, it makes the solution unnecessary complex. After a year or two nobody will remember the solution and why it was done so. It will lead to wrong decisions and unsupportable environment. The main UNIX principle - keep it simple!