This does sound like the current functioning of the invite system. As the error says, it is not intended for interacting with existing users - only new users. If you want to add existing members to a group, you want to do that via the groups page or their user admin page.
What is your process exactly and why are you sending invites to existing users? Is this a problem that actually affects your users or just something you noticed when you yourself clicked on your own invitation link to test it out?
It’s true that this is not the friendliest, most inviting response. @dan what do you think? Can we just send existing users to a specified topic and add them to any groups? I can imagine this being a handy way to reach both existing users and new users at the same time to announce a new topic or start a new group.
If not, we can just change this message to make it clear and to offer a pathway instead of a deadend, e.g. by contacting site staff.