When you invite someone to a meeting, they receive a personal (private) meeting link in their e-mail and it's only intended for them.
Only 1 user may join using that personal private link at a time. This is for security purposes to not allow others to impersonate you in meetings because when you join with your personal/private link, your name & email are linked to it. You don't want others joining a meeting as 'you'.
It's possible some of your users shared their personal/private link with others. One user can join, but any others that try to join with it will get this message.
Participants with no rights are not allowed to invite others to someone else's meeting by default. The organizer can give users permissions inside the meeting to allow them to do so through the use of a button that lets you share a meeting link that allows everyone to join.