Show Full or Closed Groups on GroupFinder
You can enable the GroupFinder to show full and closed groups. Here's how to do that:
Click on the gear icon on the top right, then from the drop-down click on Add-Ons.
Next, find the GroupFinder add-on and click the settings button.
In the settings window, toggle on "Show full groups" and hit save.
Now, check the GroupFinder to see your full or closed groups. If some aren't appearing, double check on their profile > edit tab that the show on GroupFinder is enabled for them.