I have created a custom entity and enabled Access teams. Then I added a subgrid on the form for the access team members. I used the standard documented way - Form edit - Add subgrid - All records - Users - Record's team members.
I also got a warning, that an access team template must be created. So I saved the form and Published it.
The I created an access team template for my custom entity.
But the grid displayed all users instead of just those who were assigned to the record.
The settings were correct. After some searching, I have to remove the subgrid from the form and create it from scratch. Then it started to work correctly.
So the conclusion is - before you add the subgrid to the form, make sure that you have the Access Team Template created!
I wish you all the best in 2020!