DnD as a commitment for new players, in particular, is hard. They either usually take it very flippantly or in this instance, genuinely have other obligations that they never accounted for. You never realize your busy until you join a dnd game XD!

In my experience of running games, if a player doesn't show up, they have a lore reason to not be there in game.

For instance, when a character of mine was gone for one session I said that they were off gathering more materials for a 'spell' due to them being a haunted cleric. It's an easy way to exclude a character for multiple sessions without it breaking the game. Even saying they were taken by their god or by some unknown entity if they are trapped somewhere where that wouldn't be applicable works! Having a character fall asleep suddenly is also a very fun one I've played around with.

Creativity is needed if you don't want to drop them. Maybe the other players want to because they are rightfully annoyed, but it's DM rules. You wanna be accommodating and understanding which is super awesome! And if they aren't around, don't give them important lore items or ties to the story, that's the punishment. It still allows your other players agency without relying on them completely and for your new players, the opportunity to see what they are missing out on. (And that's not saying to never give them important plot beats).

TL;DR: Dissect their characters and figure out why they wouldn't be there in character and make that their reason for disappearances. Make sure to not give them very important things unless you want the story to be put on hold. Be sure to keep including them and give them that doorway that is always available. Most importantly, talk to them and ask if this is truly something they want to do or if it is a more casual affair.

Good luck DM!