Auto-Invite Addon that you don't have to keep turning on
 

There are plenty of Auto-Invite add-ons, but I can't find one that you don't have to manually turn on every time you relog. Is it possible to have one set to always be on?

Personally, I hope that never gets made.
Why?
Because when more than 1 person uses the same invite string, it breaks any auto-inviting function. Players will get a message saying the leader "is busy" and will never be able to join a group.
And I know people will troll or forget to turn off their invite string when they're not using it anymore, and it'll cause all kinds of drama. It happens already.

It would be literally one line of code you have to change / add to achieve that, if it is something you feel you need.

I recommend using a rather unique string though to avoid the problem mentioned by NeuroticPixel.
(My personal auto-invite addon is always active and I dont have any issues)