I wouldn't feel bad leaving something like this....
EDIT:
If an owner or Mod wants me to modify this somehow, and give it their blessing, I will give you the page in Word format. Then you could share the file and let people print it themselves.
I did this in word, with an Avery template. Anyone can get a pack of 250 Cards for $10.00 - $15.00 at almost any stationary store and print cards themselves Cheap.
It's just my initial thought of what I see as something simple. I didn't want to load it with stuff talking about everything the site offers. I figured a simple picture, similar to on the site, so it looks familiar when they show up.
I Do think it should say something more friendly than "Come Join". Maybe;
You should come Join
You Should Come Visit
I just thought it was simple, and a personal note could be left on the back.....