1. Have plenty of space. People don't like being bumped when someone else gets up and they don't like having to move all their stuff to let someone else out to go to the bathroom every 5 minutes. Be generous with table space--if need be, ask some people to bring a table if they have one. They won't mind if they know that gives them more space.
2. Have food and drinks and snacks. Be upfront with people about what you will have--is it a potluck? Are you providing snacks and everyone will chip in $5? Do they need to bring their own drinks? Are you going to order pizza out? Make sure people know if they need to bring money so they can come prepared. People do not like leaving a crop to go somewhere to eat--it is better to have the food there. If they leave, they may not want to come back. Also have plenty of drinks on hand. Bottled water is a must--not everyone likes to drink form the tap so please be aware of that. Disposable cups and dinnerware also make cleanup easier.
3. Be a good hostess. You will not believe some of the crops I have been to! Make sure your guests are comfortable, not too cold or hot. Have toilet paper and a clean bathroom with a hand towel (seriously, I did not think you would need to TELL someone this but I went to a crop at this one lady's house and it was HORRIBLE). If you have kids, get a sitter or something so as not to disrupt the other croppers, especially if they are not bringing their kids. Play some music or something in the background that people will like so there are not awkward silences when people are hard at work scrapping.
4. Do games or a challenge to keep things fun. There are tons of ideas for games on here. It all depends on the group--you know what your friends will like and you can always ask them if you are not sure.
And have fun!