When you want to start an activity - and it helps here if you are seated in the same positions for most of the time - and your group is busily chatting to their friends - sit patiently and wait until they are silent. This may take a while to start with, but soon the group will become bored and realise that they are wasting time, and that there are much more interesting and fun things to get on with. Tell the group the game is called the "Waiting Game" and that you will not fight to gain their attention. That is a responsibility that they must take on themselves.
It may be tempting to start with to jump in and tell groups that you are waiting to begin, but stay seated and quiet. Soon the group will realise that that they must take responsibility for their own behaviour, and that it is not up to the leader to control the group. At all times when you are waiting you must be neutral and ignore all questions, comments until they are ready.
You must persevere with this, and gradually the time will lessen every time you meet.