I meant to reciprocate sooner but I'll try to invite some recognized members here now. If you joined in June or July 2011 and have a post count in the single or double digits, don't even bother asking. Just go away.
Quick way to invite people so they can sign up right now:
Type something in the share box and enter the emails of everyone you're trying to invite. Have them all log out of google, and go to plus.google.com. Log in, viola. Welcome to Google+.