3DS friend requests aren't auto-generated because it's Nintendo after all. So for example if you enter my 3DS code, you'll have to contact me somehow so that I can get your 3DS code entered on my 3DS.
Now for communities, I think all you need is a MK 7 community code and you'll automatically join that community on entering the code in MK 7.
Now for communities, I think all you need is a MK 7 community code and you'll automatically join that community on entering the code in MK 7.