My feeling on formals it that they the bride and groom should always be the tallest or at the top of the group.  When they are lower they are compositionally less highlighted and emphisized.

I put them first on the upper step

Then put the best man and maid of honor on their step

Then build some tall guys around the same step or one lower coming down around the sides ... leave some space between them because you will be adding bridesmaides between them on lower steps

Leave the shortest groomsman for the ends on left and right ... perhaps up one step so they are taller than the bridesmaids

Then put in the bridesmaids on lower steps - between the guys you just placed ...

You will end up with a perfect looking group ... boy girl boy girl ... with the bride and groom in the center at the top, and the bridal party or large families cascading down the steps around them.

This works with 1 - 2 steps but is best with 3 - 4 steps ... I would start with the b/g no more than 3 - 4 step up so you are not looking up at them too much.

Not so great group:

Better Group: