An introduction is for people joining in the future.

How do I get accepted as an "intro"?

After you post your introduction you should hang out in our clan chat. After a few days your introduction will be assessed where you will be either:

A: Accepted as an intro
B: Declined
C: Told proceed to apply for FA straight away

Do I have to post an intro if I want to apply for FA?

No, if you are ready to apply for FA go ahead and do so.