"Yes, I am going to invite Debbie. " 
"How do you know Debbie?"
"She is a friend from church."
"Does she live close to your?"
"No. she lives on the other side of town."
"Sometimes it is nice to have a friend ride with you. Do you think she would like to bring a friend?"

As you can see the guests are starting to multiply from the first person on her list.   Maybe the second person is a mom of one of her children's friends.  Suggest that the party  could be a great place for some of the other mom's to meet each other too.  Suggest she add them to the list too.

Maybe, she has a child on a sports team and she mentions she is inviting one of the moms.  Suggest that they help her invite all the moms so that the moms can have a fun night out socially as moms without the kids.

The hostesses first five guests can end up being 10-20 people from your suggestions by getting to know how she knows the guests she is inviting.

Let's start with some basics 

Make sure when you are talking to a hostess you let her know you are going to be dedicated to the date of her party and you are writing it down in ink.  Let her know you are going to tell your family tonight about her party and they will also be excited for me to share this date with you.  Then say, "I hope you will have the same dedication for this date for your party".