Send and accept an invitation in minecraft pe?

I have had an xbox account in minecraft pe for some time, but for some reason I can't join others who also have an account or join me in a world. We tried to come up with ideas afterwards, but none came.
Is there a solution to join others and vice versa? Why is that? And where do you see the invitation when you get one?
😅

You can see the invitation in the notifications.

Otherwise I unfortunately don't know anything about your question.