Most people here tend to go by the rule - if you decide it, you pay for it.
I agree. I was a groomswoman for my brother and his wife forced me to wear the most hideous dress AND I had to pay for it, and it was almost $400!! But I did it for my brother's sake. As groomswoman for my other brother I was told that I should just go out and find any formal black dress, as long as it wasn't cotton or jersey, so we all wore different dresses, but all in black!
Because of my experience, I'm going to have a few ideas for dresses, but I'm going to take my sister and two friends out shopping to pick their own. They might be different, but each girl will pick something within the colour scheme and we'll just coordinate with accessories to tie them together. I'll be buying their fascinators, but they'll buy their own dresses.
As for the men's suits, though.. I don't know! I think I'll let my OH sort it out, as my maids are all American and Canadian, and his ushers are English.