Attendants: Many people go into a wedding believing that the bride and groom pay for everything for their wedding party. This is not the case. When friends/family agree to be part of the wedding party, they agree to the expenses (this is why many on-a-budget couples set a reasonable maximum for attendants' attire, and have the female attendants get dresses off the rack, or at trunk shows). The bride &/or groom do pay for flowers & "thank you" gifts for their attendants.
Groom: Surprisingly, the groom (and/or his family) are traditionally responsible for more of the expenses than most are aware; the marriage license,the bride's bouquet, a gift for the bride, the honeymoon, gifts for the groomsmen, the groom's attire, boutonnieres for the groomsmen, officiant's fee, often the bar tab & (of course) the rehearsal dinner.
Bride: Lastly, the bride's family is traditionally responsible for pretty much everything else.
