What do you call people who have been invited to an event?
Solution 1:
How about Dear Friend?
friend
: a person who gives assistance; patron; supporter. Random House
: (used as a polite form of address or in ironic reference) an acquaintance or a stranger one comes across. OED
Solution 2:
Invitee or attendee would work, but these are very formal and impersonal. This is a case to use mail merge and personalize the message.
Solution 3:
How about Dear prospective guest?