It's easy. All you have to do is make sure that you have all controllers/remotes are connected BEFORE you go to Wi-fi. Once you're in a Brawl room, you cannot connect a new GC remote and when you go online, you cannot press Home to connect a new Wii Remote.
When you enter/make a room, you should have an little icon next to the other player box(es) that's the same type of controller/remote connected to that particular slot. If that controller presses start [for GC controller] or + [for the Wii Remote], then you should be able to get the person to enter. PLEASE NOTE, if you are in a room where other people are currently in, they may not choose to accept the second/third person on your end. In which case, you cannot have another person play since they won't allow it.
Also, if you enter a match in which the room says "Brawling (Waiting)", then you won't be able to have another person on your end join until after you yourself are accepted. Since you're probably not going to have enough luck on you to stay on the Character Select screen long enough until the match ends, you're going to have to either play through one match or end the upcoming match before you can get a person from your end to join the Brawl.