When you send your invitation, be honest. If you don't know this person, don't put colleague or friend, just choose the don't know option. It's fine believe me.

It's ok to keep your message simple and honest. Also, I should mention that you don't want to pre-write or create a "script" for any of these invitations. Just be yourself and be honest. You can mention something about their skill set or any other interests you notice they have to build rapport.