Ok voice of experience here LOL...

Invite:
You are warmly invited to blah blah
At [restaurant name]
On [date] at [time]
Dinner is $50 per head (includes some drinks).

Please RSVP to [names of bridesmaids] with money by [date]
Money can be given to [names of bridesmaids] or deposited into [bank details].

[contact details of bridesmaids]

I think it's assumed that you're all paying a bit extra for the bride.

You need to make rsvp date at least a week before you need final numbers. You will spend most of that week chasing people who are slack.

If you have to pay based on your final numbers, then I would strongly, strongly recommend getting money when people RSVP. Then, don't book for people who haven't paid unless you know them really well (a text a couple of days before cutoff can say "Hi ladies, just a reminder to please send through your money for the hen's night so that we can reserve a place for you"). I've heard too many horror stories of bridesmaids ending up hundreds of dollars out of pocket because people say they are coming then don't. Plus, it's one less thing to worry about on the night!

Hope that helps xo