IF you are inviting via email...
check out Punchbowl.com
It has a potluck option where you can specify what you'd like to have people bring. Then as people sign up for something, everyone see it and there are no duplicates.
DH and I just did this.
We supplied the drinks and the grilled meats. Guests brought appetizers, salads, side dishes and desserts.
There is also a place for notes (like your need for chairs), but I did not use it.
“We cannot change the cards we are dealt, just how we play the hand.”
~ Randy Pausch
"There's a difference between interest and commitment. When you're interested in doing something, you do it only when circumstance permit. When you're committed to something, you accept no excuses, only results."
~ Art Turock
"We have a saying in Tibet: If a problem can be solved, there is no use worrying about it. If it can't be solved, worrying will do no good."
~ 7 Years in Tibet