Ask what they would expect to make. Some artists are comfortable with their own ability to draw fans and will accept what the door generates. If your party is a private one, then you'll need to make them a guarantee of some kind. (divide this by the # of guests to calculate your cover charge). Let them know you'll help them vend their CD's. Explain that you are hosting a party in your home, and that they will be performing in a respectful, listening environment.
Find out if the act is a touring concern. Offer your guest room if they are travelling!