At many weddings the bride's room fills up early: her bridesmaids, her mother, a close friend or two. Whether they have their hair and makeup done there, with her, depends only on whether they want to, and it is a much requested service.
Why so many brides get ready with their bridesmaids
It changes the whole morning. Instead of getting ready alone, you are surrounded by your bridal party and your family, all getting ready beside you, and the waiting becomes a good time. You feel looked after in exactly the hours when it matters most.
It is easier for them too. They do not need to find someone on their own or rush back from a salon in a town they do not know: everything happens in one place, with one team and one approach.
How the team organises the bridesmaids' hair and makeup
While I get you ready, the team gets your bridesmaids ready. I recommend one team member for every three people, so the group finishes on time without your own turn being cut short. How it is shared out and when it all starts depends on the size of the group, as explained in what time to start getting ready on the wedding day.
Who pays for the bridesmaids' hair and makeup is each bride's decision, and there are several usual ways of handling it.