A: It's an absolute must to send a wedding invitation to anyone who received a save-the-date announcement. There's a clear expectation when a save-the-date card is received that an invitation will follow; failing to send one is essentially the same as uninviting a guest—which is unacceptable. If finances have become an issue to the planning since the save-the-dates were sent, the answer is to cut back on the scale of catering, music, flowers or invitation—instead of on people already planning to come.
Save-the-date cards are usually sent out as soon as a couple knows the actual wedding date, no matter how far in advance, to ensure that as many guests as possible are able to attend. However, this does not mean that you need to know your entire guest list at that time. It's fine to send them only to people you are sure will be invited, such as close friends and family. Later, when you have finalized your full guest list, wedding invitations can be sent to the larger group. —Anna Post, The Emily Post Institute