Shoot groups tight - no gaps, especially between the bride and groom. When everyone is in place, get them to squeeze in even more.
Have the B&G turn slightly towards each other, then have the rest of the group turn slightly towards them as well - the side profile will look more flattering.
With family groupings - have them put the rear arm round the person next to them - get them closer - depends on the family though.
Talk to them, talk to them, talk to them, keep talking when you're shooting, find someone that isn't smiling and taunt them, shoot more when people laugh.
With smaller groups, shoot full length landscape, head and shoulders landscape and full length portrait. Gives you more options later
Shoot lots fast - at least five of each group.
Work quickly - have ushers/best men getting the people for the next group while your working the current one. Get the list from the B&G before the day, and make sure they put people's names on it rather than 'Father of Bride', etc.
Advise your friend to have the minimum number of groups - every wedding I have a guest moan to me about the photographer at so-and-so's wedding that took 2 hours doing groups.
Duncan