Joining a squad
Squads are constantly looking for new players to add to their ranks. The entry requirements differ from squad to squad (the details are on their squad pages, which you can browse here), but most squads require a player to have completed the @Cadet course before considering them - the exceptions being recognised veterans that pre-date the Cadet course, and exceptional newbies who demonstrate they know what they are doing.
Squads keep an eye on players as they go through Cadet looking for skill, attitude, and willingness to learn.
Squads have been known to ask a player to join them before Cadet has finished (and sometimes before it has even started) if the player has impressed several members of the squad with a high level of ability in-game, or perhaps potential of a high ability.
If you're sick of waiting for a squad to invite you, players may ask a squad member if they can join. The squad will discuss the application privately and inform the applicant of their decision. The player may be accepted, accepted on a limited trial, told that they will be accepted if they improve X, denied because the squad doesn't want to expand, or denied.
If a squad is trying to recruit you, or you are applying to a squad, then you could well be invited to observe a squad game and/or invited on to their team on the main server. This gives you a chance to experience how the squad behaves and decide for yourself if you want to join them.