Rough idea:
You set your group name in Adalo → Triggers a Bubble workflow which creates the sharelink → Feed back into Adalo & use native share modal.
Sharelink has signup flow + explanation about MMM + link to App Store to get the app.
Onboarding field = no by default, set to yes when someone has done mobile onboarding. When new user logs in, they already belong to a group, go through onboarding & skip the group step.