Ive asked alot and ive done i think about 3 quests for them and the king keeps saying to "Play Yer Cards Right" So is it even possible to join them? If so, what are the requirements to join them?

You have to complete all of the quests for the King. After that he'll give you the option to join but you'll lose out on other options. Basically, he gives you a favor. You choose what you get. Joining is one of them.

Yes but you have to finish G.I. Blues and get the favor.

This. Once you do all the King's missions he grants you a favor. The only way you can join is to use that favor to ask him to let you in.

The primary benefit is having access to the one and only barber in all of new vegas.

Be aware, if you take the favor to join the Kings that there are other scenerios where the favor would be beneficial, like in one of the NCR quests.
Without it as a bargining chip it changes the scope of the runthrough.

^ As said above!
It depends what you need, and from whom.
If in this gamelplay you want to please the NCR you should deffinately make sure you know who you want to join and who you want to please..