Press "O" on the keyboard to bring up your social window. It will be under the "Groups Tab"
Now, anyone can do this, but the problem is you are unable to link invites to Classic chat. However, Blizz missed a channel. The "SAY" channel
Yup, it's really that stupidly simple. So, if you are standing near someone, you can invite them within Classic WOW to your community as long as you do it as a /s SAY message. Otherwise, they will need to use the invite link and copy it to a browser.
I have fully tested this and I am able to communicate from 1 account to the other. I multibox so this was easy to test, but 1 account on Classic and 1 account in BFA and the community comms work just fine.
I can't help but feel this is unintended? Does Blizz really want us to be linking to communities in Classic to invite people to?
Item linking does not work. I have tried
Of note, it doesn't send the chat to your chat window, so to read it you do need to have the Communities Interface open and communicate back and forth.