Hi @jangerrit, thanks for taking the time to go through the docs first.
The permission you are looking for the Resource is "Use."
For example, I created this "Permissions Group":
With "Use" only, my end users are able to see data on the table with a "Data source" that is querying "retool_db" to get data:
The button you see there is running a write query to add a new row with test data. As we can see from the screenshot, with this permission they can "use" that write query as well.
From reading that your users could not see the data, perhaps we forgot to click "Save changes" on the top right of the screen after updating the permissions (check the first screenshot). Changes are not automatically saved. If this is not the case, I would double check if we are editing the correct user group.
If you are still having issues, we are happy to take another look. You could join us during Office Hours on Tuesdays and Thursdays at 11am PST.