First of all pick a day, once a week when you'll have them. Picking early is to your advantage, because you can over the course of the next few months to iron out the details. Look at your friends and family and try to find a day most people can attend.
Now the downfall most people face is picking a day during the weekend, although a lovely idea, it simply never works, everyone has plans for the weekend, but Tuesdays almost everyone is free. So I know what you're thinking, it's after work or summer school, you've been up since 7 and you're tired, you're not about to drag yourself to a picnic. I will tell you this, from experience, once you start going to picnics you will find you feel less drained once you're there, and often the good feelings last until the next day.
Now pot lucking is good, if everyone brings something to the table, the responsibilities will feel more evenly distributed, however a leader in the group will often naturally present itself, this person should have a basket they never empty that's filled with paper plates, cups, utensils, napkins and a table cloth.
Alright so now you're thinking, how are all these people going to cook in the middle of the week. The answer is simple, they won't cook that day they'll bring the food they cooked the night before. By now everyone knows how to recycle leftovers, also some people will always end up going to the store and buying a little something. However the best solution is inviting people who make their own hours. My little groups and my mothers before me, have had a translator, a retired engineer, a web designer, a piano teacher, a preacher's widow, a sales rep, various students with...