First, create a test group of two users (you and a test user account) so you can isolate the problem if it’s related to group configuration and make this group viewable by all users, otherwise it won’t be accessed by all users and the rule condition would be always false.
Second, create a test form with just a panel and a label inside, to isolate any other factors that might affect the rule in your production form.
just create one formatting rule to hide the panel if the current logged in user is a group member, I know this is the opposite of what you want, but it’s easier to test this way in order to isolate condition syntax issue.
Remove your user or the test user account from the group and test.
You can set another formatting rule to show the section.
Do your tests based on that and I’m sure you’ll figure it out.