Not sure how easy this would be, given the decision to make this 3D.
(Although the 3D elements certainly don't seem to be relevant to any gameplay.)

But you might want to look towards the success of games like Among Us or Jackbox, which benefit greatly from allowing anyone with a compatible device to join the fun.
Whether that be through an appstore download or a webapp.

Perhaps even a basic stripped-down version that allows players to join (but not host).
Such a 'Guest' version might also prevent the usage of any accumulated loot and/or cosmetic customisation, to further incentivise an upgrade to the full game.

First and foremost I think you need to make it much easier for people to pull others in without having to either gift each person the game or convince them to purchase.
It would greatly help with 'spreading the word' and getting that player count up.