Write a simple adventure that starts in a village, a couple days' wilderness travel, a little bit of aboveground ruins, and a dungeon beneath that's only 2 levels deep.
Don't worry so much about deep plots, betrayers and spies on town, etc. Just get people used to roleplaying with NPCs and with each other, rolling dice, handling a character sheet, interacting with the DM for exploration, the perils of dividing treasure, the dangers of traps and monsters, the mysteries of secret doors, and the unendurable glitter of gold and magic. That's plenty to try to shoehorn into an intro module.
I also suggest splitting up the groups. Explain that it's hard to handle a group of more than 6-7 players at first.
Grab the people who want to become DMs, figure out a time slot that works for them, and fill up the party with enough players to get you up to 6 or so. Run through the adventure with them, get feedback on what they found fun or not, make some changes, enrich the dungeon.
Maybe do a second module with the same DMs and different other players, maybe the same module. Remind any repeat customers to hold off on exploiting player knowledge of the specific dungeon, but that learning from your mistakes is totally fine (for example, "hey guys lets search for traps in this dungeon").
Figure out what's important for your club. Will there be style guidelines for the DMs (for example, should DMs steer the focus away from themes such as rape, cannibalism, genocide, etc.)? Do you need to figure out some scheduling patterns for the club?
Once you have a little bit hammered out, you've probably gotten a buzz going because of the players who got to join in, and probably some players haven't been able to yet. Don't take too long getting to this point! You don't want to let the excitement fade. Get the other DMs up to speed and set up a game day where everyone can come and play, and you have 3 DMs running the intro module. Create a shared experience that everyone in the club can start with. "How did you handle the starving kobolds?", "did you try doing anything with the muddy fountain?", "how much XP did you get out with?"
I also suggest having jam sessions with the DMs to come up with cool ideas, work on one-session modules for the club, etc. But the best thing I think about D&D is that each DM has his own house rules, and especially has made up his own dungeon which is infused with his sensibilities and personality. Jim's dungeon is a very different experience from Ellen's dungeon! They require different player skills, and that's fun. So try to get just the minimum amount of standardization, otherwise there's not enough variety for players and not enough creative freedom for DMs.
Switch out DMing and playing too! You might like both, or might just gravitate to one, or get into a groove and then want to switch for a while.
Try to be supportive of people's creations and ideas.
Mainly, try to keep any bickering from festering. If someone has a problem, drag it out after the game and talk it over. Sometimes it's better to lose a player than to ruin a whole group, but recognize that there are some stormy patches in group formation. Part of being an experienced DM is knowing what parts tend to cause problems, noticing when problems are starting, and elegantly moving people along through those problems without them feeling led by the nose.