So does that mean I have to wait until the previous command be done if I want to issue another command? and if it is, I have to command every servo of a robot arm within a group move or I can only move one servo by one servo?
If you send a move, or group move command to the SSC-32 you must wait till the servos that are moving arrive at the destination before you send a new position command for those servos. But you can certainly send another move command to the SSC-32 for other servos. If you send a new position command for the servos that are moving, they will immediately start to go to the new destination.
You can also start a move, and poll the SSC-32 to see if the move is complete. This can help you make use of the time for other operations.
When a command is sent the SSC-32 immediately processes the command and is then immediately ready to accept a new command.