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)
此内容此后已更新过几次。目前的措辞是……
所有新用户必须由受信任用户或工作人员明确邀请。公共注册已禁用。
我已启用此功能。现有用户应去哪里邀请其他人？抱歉，我找不到相关入口。
为了测试，我已将默认被邀请者信任等级降至 0。仍然看不到相关选项。
所引用的“受信任用户”是指已达到 TL2（在论坛活跃 15 天）的用户。如果您已有一个现有社区，或正尝试从零开始建立社区，手动将您邀请的几位用户提升至 TL2 和 TL3 可能是一个好主意。
(
手动提升至 TL2 后，请务必点击“解锁信任等级”，以确保他们之后能够晋升至 TL3！)
你好 @shaundefense，你成功实现了吗？即创建一个独立的按钮来显示邀请弹窗？
对此有相关设置吗？我想禁用用户邀请他人的功能，仅允许工作人员进行邀请。
要么启用“必须批准用户”选项，使工作人员需批准每次注册；要么将邀请速率限制设置为 0；要么禁用本地用户名/密码登录，并要求使用替代的身份验证提供商。