I am pleased to invite you to____________________ (provide function name, date, time and location).
Include any other useful information (dress code, hotel info, what materials should be brought or will be provided).
Request their RSVP (tell you if they are going to attend or not).
I look forward to hearing from you etc etc
That is a basic outline of an invite. (it might not be formal enough...but Anglika suggests a more formal option below)