I am the leader of a Lego Club through our local 4-H. We meet at the extension office.
We meet for 1.5 hours one afternoon a month. I supply the Legos (they belong to my ds) and the kids can bring something they have built at home. I also try to bring some books and magazines we have related to Legos.
We start the meeting by letting them "show and tell" their personal creations. Then we set those aside on a separate table and open up the boxes of our Legos. They have free time to play and build whatever they want. Often they will pair up and work on projects together. Then at the end I have them share and explain what they have built. I allow 5 to 10 minutes for clean-up at the end.
We usually have 12 to 15 participants at a meeting, boys and girls, homeschooled and not.
My main role is to bring the Legos, help kids find pieces they are looking for and just watch. They don't need any instruction, they seem happy to just build.
I do have a couple of rules developed from experience: listen and be quiet while others are sharing, no play fighting or battles (too many hurt feelings) and do not put the Legos in your mouth. :)
HTH. I have really enjoyed being a Lego Club leader.