Dear (name of friend):
I am having a birthday party this saturday and would like to invite you to come along.I am turning eighteen,which is a big turning point in my life and would want all my friends and family to be there to share this special moment.It would be a casual barbecue at a park near my house and I will send you the exact address closer to the date.Please contact me as soon as possible to inform me of your attendance.
RSVP:before friday