As mentioned above, have a sign in sheet. If you can print up some sheets with basic questions such as:
Which day of the week would be best for meetings
What time would be best
Would you like the meeting to last 1 or 2 hours
Would anyone be willing to help during meetings
What they would like to get out of a monthly meeting
Name
Address
Email Address
From now through the winter months you could concentrate on how members could build different types of systems. Also if they want to buy a system you can help them out. Also during winter months members can learn about lighting for indoor systems. Also how to start germinating
seeds for vegetables or flowers. How to fertilize the water. How to check the PH and PPM in the water and how to make corrections using PH up or down and raising PPM.
Theirs a whole list of things you can do. At each meeting you should just stick to a certain topic as the members will want to come back to learn more.
Good luck!