Weddingbee boards

Depends on what you mean by "informal." 

If you're thinking fun, off-beat, not-so-traditional, then there are plenty of ways to spin the traditional invitation & wording.  These options include "Come watch us get hitched" and "We're tying the knot" - many online invitation sites (and etsy) have samples you can check out.

In order to ensure that guests know who is and is not invited it's beset to include the names of everyone who is invited on the invitation.  Then if they try to RSVP for someone who you didn't intend to invite, you're perfectly correct to say "I'm so sorry Jane.  The invitation we extended was addressed to you, it isn't possible for us to accomodate Jim-Bob."  There are many horror stories on the boards about guests crossing out (or completely ignoring) the "___  seats reserved in your honor" or "__ of   1   attending" lines on an RSVP.  It's best (and most appropriate) to name your invited guests... even the ones who are dates of your friends.

If you actually mean "we aren't having a big, fancy wedding, and I want people to expect something small at the JP/in mom's backyard/in the park" then the traditional (and probably most clear) way of handling the invitation is to write a personal letter inviting guets - instead of sending out "engraved" invitations.  Those notes would say something like:

Dear Aunt Sue,

John and I are planning a small wedding ceremony on the 6th of September.  We'll be married in our backyard, and a relaxed dinner for some friends and family will follow.  We'd love to have you and cousins Tim and Tony join us!

Please let me know your plans by August 20th.

Love, sept2013

You'd write the note by hand, on nice stationary and send it to your guests.  Presumably all your guests will know you quite well if you go this route, so it's not necessary for you to include the names of your parents, and your guests should be aware of how to contact you.  When they do get in touch you can double check just who they think is coming.  And since you'd name each invitee it should be clear that Aunt Sue isn't supposed to invite the butcher (who kind of creeps you out) who she's been on 3 dates with in the last year.

We put our first names at the top of the invite, which in my mind made them pretty informal... damn but now I can't remember the rest!

In terms of R.S.V.P. if you don't want them to bring a plus one, I think the etiquette rule is that if your name is on the envelope, you are invited, if not, it is not. On a few we put "so-and-so and guest" right on the envelope (we had only outer envelopes, no inner).

It didn't actually work though because one person who I didn't invite plus a guest put "2" next to "number of people attending", but that was just an oversight on my part, I really didn't expect her to be bringing anyone since she is flying from several provinces over, but I guess she is bringing a friend from the city I live in (she used to live here too). Shoulda thought of that.

The best thing to do, I think, is tactfully contact each of your single guests individually and ask them if they want to bring a guest. You can use the excuse that you just need to know what names to put on the placecards and envelope and stuff. We did this, and many of our single friends (graciously!) said they were not bringing anyone. A couple of people asked us to just put 'and guest' which I guess meant that they did want to bring someone but they weren't seeing anybody seriously enough to know if they would bring them, or maybe just a friend or something. We don't have that many single friends so it wasn't a huge ordeal but that worked well for us and I think it was pretty polite.

@Casimir23:  I love that, how cute!