In the meantime I’ve found a solution with action and lookup function in google sheets inspired by some tutorials by @ThinhDinh
How I do it in a app where customers can order items from sellers:
when the user orders something through an action a new row in table orders is created and adds his email to the a row owner column and the seller’s rowID (which is linked to the product anyway) to another column X.
then an array function in google sheet takes value in X and pulls the sellers email through a lookup in the table sellers into the second row owner column, hence this order record is only visibly by the two parties
@Robert_Petitto I think this could also apply to private chat messages and the email data of all people can stay in the sheets.