Is there an appropriate way to word this emailed invitation?

My sister is having a baby in May and since she has a two year-old she is not interested in having a shower. I asked if she would like to celebrate this baby with a girls night out to dinner and she liked the idea. Most of the women attending are family and I am totally comfortable telling them that I would like everyone to pay for their own meal, and I will cover the mother-to-be and provide some kind of special dessert. Honestly, unless we get pizza from Little Caesar’s I cannot afford to buy everyone dinner. There are two women I don’t know very well and I want to convey this information to them without sounding like a total cheapskate. Any ideas for appropriate wording for the invitation?