Hello,
Sorry to hear you are having issues. There is no user manual that I know off. Everything, is pretty much straight forward. Just try to group two explorer together. Click and drag one of the explorer titlebar on top on the other and a notice should come up on the left edge of the bottom explorer titlebar "Group this window and the dragged window" If you release your mouse button than it should group. And you can see the 3 dot menu buttons. Click on it to get to setting. You can manage how Groupy work for you from there.
If that does not happen on your computer, please try uninstall, re-download the latest version from your account and reinstall again.
Report back here.
Basj,
Stardock Community Assistant.