There are two possibilities for a capoeirista to enter the roda and to play.
The first one is, if two players kneel in front of the principal Berimbau, shake their hands and start with an Aù (cartwheel) into the roda. When their game is finished they shake hands again and move fastly out of the circle to give it free again. It is to observe that you should never stand with your back to the center of the roda, no matter whether you are standing inside or outside, especially when you are leaving after your game. This is about safety - maybe another player has not realised that you haven't left the roda completely before he starts with kicks and jumps. By the other hand you should be cautious where the other players are when they leave the roda before you enter.
The second possibility to enter the circle is to "buy" the game, a so-called "compra". During normal trainings or training rodas (usually no live instruments, no singing) all cords can buy the game. In an official roda only higher cords can buy the game. A lower cord may not buy out a higher one, except for the case that the Mestre or the Professor who is conducting the roda allows you to. If you enter like this it is very important that you make sure both players can see you. Approach from the side and be distinct in your movement.
There is an etiquette to buy in the roda which is hard to explain... just observe the higher cords and do not hesitate to ask, too.