I do not have other admin functions besides "Add User". What do I have to do to get the functions like remove user?
Depending on your plan and/or setup, different functionality is available. You can read about the various Admin Roles here: https://helpx.adobe.com/enterprise/using/admin-roles.html
So for example, if you're a Product Admin, you can un-assign a user from a product or product profile, but can't really delete a user. To remove a user, you'll need to select a user and only then the option will become available.
However, if this is not working as expected, then I'd suggest you ask your Sys Admin to investigate and/or open a Support Case.