It appears that this got completely changed in either our 5.0 or 5.0.3 upgrade so that it now tells people they have to go through a registration process before joining a group. This message sounds appropriate for an external user, perhaps, but if you are using the user-picker to select people to invite, it isn't correct information and is very confusing to my users. I want to update the text, but don't have a copy of what it was before we upgraded (in April).
Please and thank you!
I've emailed them to you