I am not quite clear on what you are trying to do here, but we discussed users and groups in an earlier thread.
For MineOS there are no “correct way of having users”, since MineOS (or the WebUI) it self do not handle users or groups. This is entirely handled by the OS (for the stock Node MineOS this would be Turnkey Linux, which is a heavily tweaked and slimmed version of Debian, i seem to remember).
It sounds like you are trying to host a Minecraft Hosting server (or a “Rent-A-Minecraft server”). MineOS with it’s WebUI is not such a system. There are other minecraft managers out there that do that much better.
MineOS is really meant to be a WebUI that enables one, or a group of, administrator(s) to handle one or more Minecraft servers as easily as possible. Since MineOS respects, and uses, the underlying OS’s users and group system, it is possible to set up a system where different admins see different systems, but as you seem to have discovered, you need to keep your user-groups straight to do so.
I guess that by adding a user to the mc group, then removing that user from the group matching the username, you would force all new servers to be owned by the mc group. ou would also then need to change the group ownership of that users home directory to that of mc. This again would mean that the private home directory of a user would no longer be private, since the entire mc group would be able to access everything belonging to that user. So this is really not good user management.
As I told you last time: It is reccomended to stay with one user (the mc user) until you get more experience with linux, and with user management.
Remember that each user you give access to MineOS will have administrator access to your MineOS platform. You do not want this. You want as few admins as possible. You do not want others to start tweaking java arguments due to something they read on the internet, or to give their server just that little more memory, since their server feels laggy. This will lead to crashes, lag and other problems. Once a minecraft server is configured and running, all you really need to do for it in the WebUI is stop, upgrade, start. MineOS can handle backups and archiving automagically. MineOS can also automagically start your minecraft servers when you start your MineOS server.
I have a MineOS server iwth 7 active minecraft servers (3 for pocket edition, 4 for standard edition), and I am the only admin. I seldomly have to access the WebUI unlesss I chekc something before posting here, I need to start a stopped server, or I need to update something. I do not need several admins to do that. My users tell me when something is amiss, and i need to do something, and usually I can do so easily in a few minutes (log on, open the correct server, press start).
So I’d say: the best way? Don’t. Be the only user.