One issue with lobste.rs invites is they are a mechanism to vouch for a person. Whoever invited you will show up in your public profile. So there’s a reputation risk if you invite who later gets a lot of negative attention. (Which is why I’ve yet to invite anyone.)
Edit: Here is the text for that.
Invitations are unlimited, but persons you invite will be associated with your account in the user tree and you may be responsible for them if they cause problems. Please use your discretion when inviting persons you don't personally know.