Invite codes worked fine for Gmail, but you weren't limited to only the people on Gmail to talk to. It was a full, regular email service. You could email anyone and receive mail from anyone. I doubt it would have been very successful if it was invite only and you could only email other Gmail users for the first few years.
Waze was also invite-only, G+ was initially invite only. Did that model help or hurt them?
I think it helped them. Gmail had more trouble with invite codes because some people wanted a Gmail account, but didn't know any existing Gmail users, because Gmail was useful for communication with non-Gmail users.
G+ didn't have that problem so much, but I don't remember it using invite codes.
Waze was also invite-only, G+ was initially invite only. Did that model help or hurt them?