The One Piece manga has been revealing Luffy back-story since Chapter 582. These chapters describe the relationship between Luffy, Ace and their friend Sabo (pictured above).
[STORY] & [SPOILERS]
Sabo is the son of a noble but decided to run away from home to live with Ace and Luffy. He hated the fact that his life was being controlled by others. Eventually he is captured and returned to his parents. Later the nobles decide to burn the area that Luffy and Ace live in and Sabo is distraught. In an attempt to get help, Sabo runs into Dragon (Luffy’s father).

[THEORY]
So, here’s what I think. Sabo meets Dragon. Dragon takes him in or helps him achieve freedom. Then, in the current time, Sabo helps gather Luffy’s crew after meeting one of them and hearing their story — probably Usopp because he can’t keep his mouth shut. Once the Mugiwara gang is gathered; Sabo brings them to Luffy (maybe on Amazon Lily, maybe not). He then proceeds to bitch slap Luffy until he gets over Ace’s traumatic death. (Oda Style)
Since Sabo said he was going to be a captain in the back-story, I’d say the chances of him becoming a crew member are low-to-no. Also — (continued after picture)

– Sabo is not Law. I promise. To begin with, Sabo is from East Blue and Law is from North Blue. And the addition of Dragon into Sabo’s back-story makes him being Law even more silly. It’s not happening. Sorry. Feel free to leave ideas in the comment section.