Can I use built in system groups for privileges or do I create my own?

I'm new to OMV but I have a little bit of general Linux experience and I think I'm having a conceptual issue with how OMV permissions works. I have a one day old install of OMV 6 on a Raspberry Pi 4, as far as I can see everything is working correctly. I have a couple of shares configured and I can access them and have the permissions I would expect.

The problem... When I installed the Pi I created a user Alice, Alice has a self-named group. Through OMV I created a new user Bob, Bob didn't get a self-named group. My first question, is it ok to delete the Alice group, it's not doing anything useful as far as I can see but it shows up in all the share privilege lists.

Now for the thing I really don't understand. On one of my shares I'd like to assign privileges based on a group. I had expected the system created group "users" to appear in the list but it doesn't. I can add users to the "users" group but I can't then give that group RW privileges. Should I create my own group, called for example all_users, though OMV and manage it that way?

I did notice, however, that all the users and all the system groups are available under the ACL settings. Should I just give the "users" group RW permissions here? Is this the same making the setting one the privileges page? I don't think I'm understanding the difference between privileges and the ACL.

Thanks for any help.

Thanks, that's a good start and what I was hoping to here. I think I have pretty simple requirements and the ACL looked like a sledgehammer to crack a nut.

So do I have to create my own "all_users" group through OMV in order to set pivilages?

I created a test user and they were indeed automatically added to the "users" group. That's great but the "users" group doesn't show up in the privileges list as you can see in the the attached screenshots.

My question is how do I assign privileges to the group if it doesn't show up?

Thanks for taking the time to come back, I've been experimenting today and that was the conclusion I came to.

The way I'm understanding at the moment is that the "users" group (gid=100) is being used for file level permissions. I can see the permissions on the share I've created are 2775.

The "all_users" group I create though OMV is for an additional permissions layer on top of that which can be used to control access to the shares.