I'd avoid AL for your situation. If you haven't already read the FAQ, AL Dungeon Master's Guide, and AL Player's Guide, I highly suggest you do so before further considering it. For one thing, I'm not sure a game you host at work qualifies as "open to the public," but I don't know what position AL admins would take on that.
If you do pursue this avenue, I'd get started with DDEX or DDAL adventures, specifically the ones intended for level 1 PCs. There's no real difference between EX and AL, it's just a naming convention (Season 1-3 = DDEX; 4+ = DDAL). The level 1 modules are bundles of mini-adventures each lasting about 30 minutes (in theory), with PCs usually reaching 2nd level after completing 3 or 4 of them. Other DDEX/AL adventures last 2, 4, or 8 hours (again, in theory) and are self-contained stories. However, they are meant to be completed in a single session. This won't be too problematic if you have the same players for split sessions, but if there's any variation... Well, it's been a couple years since I ran AL, so I'm not sure what the current official guidance on that is. At the time I ran, the advice was basically don't do it. It's logistically difficult to award XP and treasure fairly when the players change mid-story.
Instead, after completing the mini-adventures, I'd switch to a hardcover adventure path (DDHC). Rotating players shouldn't be as problematic, but you might still run into some murky territory. I can't give any recommendations for a good starter AP, I'm afraid. I ran pieces of HotDQ and PotA when another DM couldn't, but most of my DMing was for Expeditions. Read reviews for the hardcovers and pick one that interests you (and hopefully the players).
That said, the biggest reason to play AL, in my opinion, is for character portability. If everything is on the up and up, the players should be able to take their characters to any other (tier appropriate) AL game in the world, and even come back to yours for more, all without raising questions about the legitimacy of rewards. If that's not the primary motivation for your players wanting to do AL, I strongly recommend avoiding the program altogether. You can still use DDEX and DDHC adventures even if you aren't participating in the program. You'll also have the freedom to modify the game as suits your needs. The only thing you'll miss out on is the aforementioned portability.
(edit: apologies if I've made any factually incorrect statements about AL. Haven't been in the program since the end of season 3 so I'm a bit out of touch. I welcome corrections)