Took about a month break and just learned about these.
-Is 391 good enough to get into one?
-Do you need to get into/form a full raid group to start one?
Best thing to do is to create your own group, that way you don’t have to worry about anyone else’s requirements and no one’ll judge you on your ilvl.
It requires you to form a raid group of 30 players max and it’s ideal to form up a group of ilvl 400+ players for best results. Just do that and let the raid run itself, as most everyone’s most likely done it by now!
Heroic Warfronts really should’ve been released at 8.2’s launch, as the difficulty is welcome and it’s really easy to lose if the raid doesn’t follow mechanics.
I did it once so far in someone else’s group and found it to be super easy. BUT with that said, it’s a premade group, subject to e-peen elitism like any other such groups (except for world bosses). So unless you’re a healer or you have the patience to make your own group, I suggest you grind up your gear a bit as most people wouldn’t accept a dps with less than 410 or sometimes even 415.
I can give you some pointers if you need on how to gear up without raiding.
You won’t be getting into as pug group with that, better start your own. I actually just finished one, I’m one of those jerks who does 425+ only for a fast game.
Much appreciated all, thanks. 
Took the plunge and signed up for one, surprisingly got in at 391. Was a lot of fun, I enjoyed the slower pace, greater threat of danger and the actual feeling that my gathering work paid off (spent about 85% of it chopping wood hah).
Got a nice 430 staff to replace this 370 one.
No you will be useless id say like 405-410 if youre playing optimally…
Lucky you! I’m happy you were able to get in a group.