We are currently working on a APP where every user can create multiple institutions and every institution has a monthly or yearly price, but this mean that every user can have multiple subscriptions within the same group! We didn't find a way to do it using renewable IAP subscriptions, in this case, does Apple allow us to use third parties as Stripe?
Considerer the same question to Google services
I didn't try anything yet.
Stripe has a canonical documentation here that explains when you can and can't use them versus IAP. It's about what you are selling and whether it's a physical product (groceries, gym membership, etc.) or not.
This decision is not something Stripe has any control over and is controlled by Apple directly based on your business model. You likely want to contact them directly to confirm whether you have to use IAP or not.