Large group shots aren’t especially easy to arrange, first of all you’ve got to corral all the people you need together (and get rid of any unwanted hangers-on), then you’ve got to make sure that everyone is visible, smiling, looking at the camera and not blinking.
The ushers can usually be relied upon to help find everyone that’s supposed to be in each shot, but it’s up to you to arrange them.
Put the most important people towards the centre of the group around the bride and groom and have the taller one’s towards the back of large groups.
Bring a stepladder and tall tripod, or find a high vantage point for shooting very large groups.
It can be useful to bring something along that makes a bit of noise to attract attention – a whoopee cushion can be relied upon to create a few smiles, but it won’t work more than two or three times.
One way to avoid having people blinking is to get them to close their eyes and open them on the count of three, when you take the shot.
Take a few shots of each group in the hope of getting everyone looking as you want in at least one, but be prepared to do a little post-capture compositing.