The name comes from the french word for buttonhole. Boutonnieres, as you know, are one or two small blooms pinned on to men's jacket lapels. For weddings, all of the men in the bridal party wear one. So, of course the groom gets one, the father of the bride, the father of the groom, and all the groomsmen. If you have other special men in your wedding party, like a ring bearers or greeters, they should have a boutonniere too. And if you are lucky enough to have your grandfather there, it is nice to honor him with a boutonniere too.
It is always a nice touch to have the groom's boutonniere play off the brides bouquet. For instance, if your bouquet has roses, calla lilies, freesia, and rananuculus in it, then maybe the groom's boutonniere could be made with a calla lily and the others' boutonnieres could be made with freesia. Here are some of great ideas for boutonnieres.