1. Is there something wrong with those squads?
2. What's the best way to join a squad?
1. Nothing, but if they are desperate enough to ask anyone Tom, Dick, or Harry to join, there might be a reason they are so low on numbers. They might be low in numbers, have no cohesiveness, or have no recruiting practices.
2. Fly with a group of guys for a couple weeks to see if their ideas for the game fits yours. If not, no hard feelings if you keep looking. You'll find some of the better squads don't openly recruit or do so by invite only.