As long as the players can fit through the neck of the bag they can exist inside until you’re nice enough to let them out, thanks to the necklaces of adaptation.
Even without them, it would still be possible but how long they can hold their breath would then become an issue.
Just don’t die because no one else will know to let them out…
I think using Bags of Holding like this is always a great idea. My players have used a BOH to fly team members across chasms, hide them from magically tracking hunters, sneak extra people into a party, etc.
It can subvert a lot of what the DM thinks might be a challenge for the players and turn tricky situations into slight inconveniences so some DMs may place limits on what can be done.