i did that partially. I have a bit user subscribed to many communities. Unfortunately that is a fully manual task - currently i don’t have an automated way of doing this

Somehow you are describing an „email“…

Problem with you approach is, that you have an easier maintenance, but your users need an additional tool. Especially when grandparents should use this, you should stick to what is there.

In theory you could host a matrix server and install some bridges like whatsapp, discord, etc. probably you would be able to create a matrix room and invite your granny via the whatsapp integration, your friends via discord, etc.

Disclaimer: I did not set up such solution - this just came to mind for your situation.