'Bridesmaid or best man' ?

Neither do I, and I have been on boards awhile. Except perhaps that sometimes when a man has a woman on his side, some people don't know what to call her, though Groomswoman is common. And when she is groom's best friend like a Best Man, ? What to call her. Similarly, some people don't know what to call a man on the bride's side, usually a Bridesman if with the bridesmaids, but then what to say if he is in place of MOH? Man of Honor, Best Bridesman ? People used to fumble around a lot over those, calling them " my Best man Bridesmaid ( my best friend is a guy) and stuff like that in posts. Those have appeared a lot, as people have figured what to call whom, when a woman is on groom's side, or man on bride's, or sometimes in a sec sex wedding. Confusion. For example, a couple weeks ago someone was trying to describe a relative's wedding, same sex, where one was in traditional bridal clothes and one in groom's clothes, and each had men and women standing up with them. And the Original Poster tried every combination of groom's bridesmaid and whatever, trying to be polite, but funny to read. The next day it was gone, and a replacement post up that was clear, and did not ( accidentally) come off making fun of people, as the first had.
But if that is not what you are referring to , I am clueless too. And I have been on and off boards since my first marriage 20 years ago. When they were moving from local newspapers to online websites.

Honestly the way I see it is they are the people in charge of everything (bridal shower/ organizing the bridesmaids/ groomsmen, making sure the bride/groom is prepared

Do you have an example? I’m pretty active on WW and I’ve never seen this before.

A “best man” would be like a bridesmaid but for the groom.

Are you on Weddingbee and/or The Knot?

It doesn't happen on WeddingWire, but on other forums when people use the abbreviation 'BM', the forum autocorrects it to 'bridesmaid or best man' because it doesn't know which one you're referring to and people, especially outside of the wedding world, get confused by all the acronyms.