From the main window, select the group you wish to export. Then, go to the File->Export->Current Group... menu. You will be prompted to choose a filename. The default extension is .Group, and it is recommended that you use this for all groups. Each creature in the group will be exported into the single .Group file.
If DM Genie is not running, double-click the file and it will start up and import the group. If DM Genie is already running, from the main window go to the File->Import menu. You will be asked to select a file and then click “Open”. You will then be asked for the name to give the new group. You can keep the default or rename it as you like. If the group came from a different campaign, you may have some Unknowns Found During Import .