You can join a V15, get your unlock if you win, and after that keep joining ppl on V15 having them open for you and gain RP. That win will let you augment up to R20.
But you won't be able to open the fight yourself until winning all former tiers on V15 :)
You need someone who has access to V15 to take you in for the clear and then to take you in to farm RP. It will unlock the rank for you. But you yourself can not take anyone into V15 Bumba unless you have cleared V15 of all of Atonement 1, 2 and 3. Now you can have someone with access take you for the clear on all these V15 fights and you will get access to take people into V15 Bumba yourself.