NetGore

From what I remember, invites do time out after a while (defined in the feature's settings). No declining.

Not sure why overwriting an invite "isn't a good idea". Sure, there are plenty of ways it could be better, but this works plenty fine. Only time it'd be an issue is if someone else invites you to a group right before you accept and you join the wrong group, but its not like you can't just leave and join the other group.

Feel free to change it to that way if you want.

So invites are overwritten by default?
Where's the timeout defined?

EDIT:

Found the place it is defined.

Yep, overwritten by default.

Okay, one last thing I noticed there is no "group" channel.

I created one for admin channel but I was wondering if you could proofread it so I know it works as expected:

In Demogame.Server.World.cs

        public void SendToMods(BitStream data, ServerMessageType messageType)
        {
            if (_users.Count <= 0)
                return;
            foreach (var map in Maps)
            {
                map.SendToMods(data, messageType);
            }
            foreach (var map in InstancedMaps)
            {
                map.SendToMods(data, messageType);
            }
        }

And in Demogame.Server.Map.cs

        public void SendToMods(BitStream data, ServerMessageType messageType)
        {
            if (_users.Count == 0)
                return;
 
            foreach (var user in Users)
            {
                if(user.Permissions == UserPermissions.Moderator ||
                    user.Permissions == UserPermissions.LesserAdmin ||
                    user.Permissions == UserPermissions.Admin)
 
                    user.Send(data, messageType);
            }
        }

Is this the correct way?

Almost. It'd work, but not if you use more advanced permissions since permissions are bit flags so you should be checking the bits. Since Admin and LesserAdmin imply moderator, just do:

if (user.Permissions.IsSet(UserPermissions.Moderator))

Rest looks good.

What is level? Doesn't exist in the code.

Sorry, replace level with "user.Permissions".