In order to install the Grammarly add-in for "all users":
- Launch the grammarly Add-in installer
- Press and hold the CTRL + Shift keys.
- Click on the Get Started button
- Release the CTRL + Shift keys
- Check the Install for all users checkbox
- Click on the Next button
If it doesn't help, please temporarily disable antivirus and anti-spyware software and repeat the process described above. Once you've finished the installation, please enable back your antivirus and anti-spyware software.