I have hosted casual outdoor parties for over twenty years now; for the last ten years my wife has decorated and cooked for them. There have been six marriages (including mine) between people who first met at these parties. They are becoming legendary so perhaps it is time to share some party lore.
Two keys to party hosting:
You need to provide:
Let guests come and go as they please and bring whatever they like to eat or drink. People are shy in a new place, many will hang back and watch for a while. Just welcome them then let them be. Avoid planned events or scheduled activities and have as few rules as possible. Encourage conversations between people who would otherwise never meet.
Go for diversity; different ages, incomes, educations, etc. Most of the guests should be strangers so there are no in groups. People will still talk mainly to people they already know but this can't be helped.
There are many ways to tell people about your party; publication in a local magazine, phone calls, direct mail, email, posters, leaflets, etc. Phone calls are time consuming but they have the highest response rate. Email is cheapest and quickest. If your next party is scheduled encourage guests to invite their friends by giving them extra invitations.
Your invitations should say "Please bring food or drink to share". Most of the food and drink should be brought by the guests. People will call and ask what to bring; avoid specifics but describe the type of food and drink that is usually present. There will always be people who bring nothing; let them. If guests want more food and drink encourage them to take up a collection to go get more. If someone has special food or drink that they don't want to share with the crowd they should keep it separate.
Your invitations should say "Friends, family, and acoustic music welcome". This makes it clear that people can bring parents and children. The age diversity will loosen people up and keep them peaceful. Invite people to bring their pets if you can deal with the unexpected. Acoustic music is good because the neighbors have to sleep, loud music inhibits conversation, and the guests can create it themselves.
If necessary post a sign on the entrance: "Government spies are everywhere! No underage drinking or illicit drugs on party premises." You may have to expel guests who won't cooperate.