I would talk to each girl, individually may be best, and find out what she is planning to do regarding hair and makeup. Some of them may go to a salon for their hair but opt to do their own makeup, and some may assume a makeup artist is being hired for everyone to use.
Once you know everyone’s situation figure out how much hair and makeup will cost per person and if it’s too much for each bridesmaid, or maybe just for a few of them, see if you can subsidize some of the costs. If you can’t, shop around for the best deal. Maybe there’s a beauty school in the area with some talented students or an assistant to your salon can send you at a fraction of the stylist’s fee. Or even a groupon deal you can take advantage of.
Another option that I did for a friend’s wedding was to go to the local department store with a few other bridesmaids the morning of the wedding. We each had scheduled an 11am appointment at our favorite makeup counter and for the cost of one lipstick, which we bought to use for the wedding, got our makeup done for free. I honestly don’t remember what we did for the hair. I think for my wedding we hired a makeup salesgirl from the local Macy’s to come to the ceremony location and do each girl’s makeup for about $65 each. For hair, a friend found a local salon that gave the girls a break because there were 8 of them coming in for blowouts at 9am on a Sunday.
Keep your bridesmaids’ budgets in mind -- they’re spending a lot of money on your wedding -- so don’t force them to lay out hundreds more for hair and makeup. It’s worth it to start researching budget-friendly options months, and not days, before the wedding.