At the start you have one hero which you can level, equip and fight with. Later in the game you are able to unlock additional hero slots for your team. Your team can have up to three active heroes. They will fight for you together as a team.

You can find more heroes on quests or you can recruit them.
Recruiting offers heroes for money. There are limited heroes every hour for the whole server. If someone buys one of the heroes it is gone and can't be recruited by an other player.
Recruiting will be changed until the main release though. We are not 100% certain in which way.