Invitations can be confusing things. Two envelopes. When do you ever send something with two envelopes? If you don't handle these things correctly, each of these can hold conflicting information. If you want your guests to be very clear on who is and isn't invited, be clear with the invitations.

Here's how they work:

Outer envelope - addressed to the householder.
Inner envelope - the names of each person invited.
RSVP - some people think the "number of guests" line is confusing, because it might suggest that the invitation is open to more than the number of guests listed on the envelope. However, if the invitation was made out to four people but only two can attend, this is necessary information. Personally, I rather like that line.

What if you aren't using an inner envelope? Lots of couples aren't, either for cost-saving or for environmental concerns. Then you put the names of all invitees on the outer envelope. And then everyone is clear on just who's invited. Simple.