If you have multiple monitors, you can also try the following to resolve the issue:
1. Change your Display Resolution to a larger resolution with Revit running and the Group Editor active.
- In Windows 10, right click on the desktops and click "Display Settings"
- Change the "Display Resolution" to a different resolution (preferably a finer resolution)
- Do this for all monitors
- Minimize all apps and look for the Group Editor dialog box on all monitors.
- If you can see the Group Editor" dialog box, move it to the center of your screen and then you can reset the "Display Resolution" to the original setting.
- If you cannot find the "Group Editor" dialog box, reset the "Display Resolution" to the original settings and try method 2.
2. Change size of text, apps and other items. Again do this with Revit running and the Group Editor active.
- In Windows 10, right click on the desktops and click "Display Settings"
- Find the "Change size of text, apps and other items" drop down box and change this to a smaller percentage (I had mine set to 150% and this was causing the problem).
- Do this for all monitors
- Minimize all apps and look for the Group Editor dialog box on all monitors.
- If you can see the Group Editor" dialog box, move it to the center of your screen and then you can reset the "Change size of text, apps and other items" to the original setting.
If these don't work you may have to reset the RevitUI.
https://knowledge.autodesk.com/support/revit/troubleshooting/caas/sfdcarticles/sfdcarticles/In-group...
Good luck!