This is a bug in 18 up to 18.0.1. It may be fixed in 18.0.2 but it doesn’t seem clear from what I can understand of github.
The workaround is to create another admin user and use that to edit your own admin user profile. Then you will be able to add yourself to the family group.
I agree the contacts list under the icon on the top right is pretty brain dead. I think if you are in a group it will show other users in the group and any you have shared with but it seems pretty random. All you can do is email them. I routinely hide that icon.
The contacts app is your personal list of contacts, you need to add people to (I think you can upload and sync with cardDav but I haven’t tried. You should also be able to share contacts with a group or individuals so create a shared family contacts list and sync to your devices - again I haven’t tried.