Here’s a contrivance. You could allow anyone to join, but have no read-only categories. People would have to be added to a group to be able to get to the Good Stuff. People would have to know to join a group to see the stuff.
You’d need to think through that, but you could assign lots of people to be group owners who’d be able to add more users.
Or just create a “please invite these people” topic, in a category that only TL2 members can see. If people post in they are TL2 by definition (or set up whatever category permissions you prefer), and staff can edit the request posts to put a 
next to those they invited.
Thanks for the suggestions I’ll have to talk to my partner and see what he thinks.
If anyone happens to think of any other possible solutions I’d be happy to hear them!
I am totally confused with this topic. Ignore me if I am wrong.
Anyway just now I tested invite functionality in my localhost and it is working fine with below criteria.
invite only SiteSettinglogin required SiteSettingNow I am able to send invitation as that new user. Even it is working while both or either one SiteSetting disabled. I can’t understand why it is not working for you. Do you have SSO enabled?
Currently only TL2 users can send invites. So no harm when we welcome friend-of-a-TL2-friend-of-a-TL2-friend-of-a-TL2-friend 
Dunno, the OP will need to confirm if SSO was enabled.
SSO is not enabled nor is login required.
What about enable local logins SiteSetting? It says “Enable local username and password login based accounts. (Note: this must be enabled for invites to work)”
Update: must approve users SiteSetting doing it. While it enabled staffs can send invites. But not normal users (any TL user) can’t.
Isn’t that supposed to be done through invite only AND must approve users?
It’s possible @vinothkannans is right here – must approve users.
Clearly the interaction between these settings is complex and confusing.
@vinothkannans got it sorted out for us it was the must approve users. setting.
So I guess my next question would be is it possibly to add a button in the customize html section that would open up the invite modal?
Okay let me explain when all the invites won’t work for a user.
must approve users setting enabled. (staff are immune to this limitation)This has been updated a few times since. The current wording is…
All new users must be explicitly invited by trusted users or staff. Public registration is disabled.
I have this enabled. Where would an existing user go to invite another person? Sorry, I cannot find it.
I’ve lowered default invitee trust level to 0 for testing. Still not seeing it.
The referenced “trusted users” is users who have achieved TL2 (15 days returning to the forum). If you have an existing community or are trying to bootstrap one, manually granting TL2 and TL3 to a few users that you invite may be a good idea.
(
make sure to click “Unlock Trust Level” after a manual promotion to TL2, to ensure that they are able to earn TL3 later!)
@shaundefense 님, 안녕하세요. 초대 모달을 표시하기 위한 별도의 버튼을 만드는 데 성공하셨나요?
이 기능을 설정할 수 있는 옵션이 있나요? 사용자가 다른 사람을 초대할 수 있는 기능을 비활성화하고, 스태프만 초대할 수 있도록 하고 싶습니다.
‘사용자 승인 필요’을 활성화하여 직원이 모든 가입을 승인하도록 하거나, 초대 제한 속도를 0으로 설정하거나, 로컬 사용자 이름/비밀번호 로그인을 비활성화하고 대체 인증 제공자를 요구하세요.