Are there such roles? If so, what purpose do they serve? If creating bridesmaids or groomsmen is a distinctly American (or Christian) cultural practice, then what are some reasons for or against including it an American Jewish wedding today?
The Talmud talks about having shushbinin -- close friends -- escort the bride and groom, to the point that someone who was shushbin at your wedding can't testify in court about you, as the personal connection is too tight.
What is still found today is having one good friend (each) serve as "honor escort", (shomer) for the bride/groom: for a day or two before the wedding, the bride-and-groom-to-be are treated as VIPs such that they shouldn't go out unattended. Hence a good friend accompanies them until the wedding.
Today I'm not aware of "bridesmaids and groomsmen" being part of what we'd expect at a traditional Jewish wedding, but given the above Talmudic discussion, I can't get all that upset about it. (Though keep in mind, we're talking a very tight handful of people whose job it is to help out, not a massive chorus line to serve as window dressing. Similarly, a lot of people will likely be busy, or don't want/need the spotlight. A Jewish wedding is about things that are bigger than just ourselves. "Hey everybody look at me!" isn't the right theme.)
As for the bridesmaids all wearing matching dresses -- I'm told this has non-Jewish origins about so the demons can't tell who's who or something -- my guess is no one today thinks superstition, only aesthetics on that one.