Adding an email to a group via invite

It’s nice that if I upload a ‘bulk invite’ csv like this:

nice@test.com,nicegroup

If a user with the nice@test.com email already exists, they will get added to nicegroup.

However if I do an invite that isn’t a bulk upload, via the api or web interface, then if that email exists, they won’t get added to the nicegroup. Is this by design?