I recently helped write a set of Contra Etiquette tips for CDNY that might have some useful stuff: http://cdny.org/what-is-contra/contra-etiquette/
A lot of it is geared towards more experienced dancers, but you could pick and choose relevant points (and maybe de-technical-ify the language some) that seem like they apply to beginning dancers.
For what it's worth, in my pow-wow with the beginners just after the lesson ends and before the dancing starts, I say the following:
- anyone can dance with anyone (that goes for genders and for experience levels). Anyone can ASK anyone to dance (so new folks ask experienced, women ask men, men ask men, etc.). Anyone can decline an offer to dance.
- if you mess up, smile and laugh--it's really fine! (Usually I point out that the experienced folks make 10x more mistakes, we're just better at hiding it)
- contra is full of figures that are for the most part very simple--the tricky bit is explaining the figures in words, and then matching the calls with the figures, but I promise you can do all these figures. (And it gets easier with practice!)
- the rule in contra is "better never than late"--if you mess up one figure, shrug and smile and get in place for the next one, rather than trying to catch up
You might consider saying something like, it takes a few dances to start feeling really comfortable with this stuff, so we hope you come back again?
And of course, I recommend writing in big letters, "we love you, come dance with us again!"
CDNY gives "your second dance free" passes to first-timers--is this something your dance might consider?
There are a bunch of rather disorganized thoughts--I hope some of them are helpful!