I will clarify some things:
There are at least two different kinds of crews:
1) Competitive Crews
2) Casual Crews
For Competitive Crews:
A competitive crew will usually only accept members that are in the same skill range as them.
People have better chances of being accepted if they play a character that no one else in the crew plays. Crews don't accept usually annoying kids unless they are really good.
Crews are almost always formed based on location. They want to be able to practice together often.
Asking crew members if you can be part of their crew is probably not a good idea. If you want to join a crew, try and get together and practice with them as often as possible. If you are good enough, they will want you in their crew, and ask you.
For Casual Crews:
This kind of crew is based on friends who play together all the time, who decided to name their group.
Often, not all members of a casual crew will go to tournaments, and the skill levels of each player in the crew is extremely varied.
When joining a casual crew, it's friendship first. You have to basically just be really good friends with everyone else, and play with them all the time, and then you can usually just assume you are part of the crew. Asking to be in a casual crew is fine.