Right-click the Name field of that item and from that menu click "Move To Archive".
(If you click the subject field it will give you a totally different menu).
To look at your Archived messages, click File on the main window Command Bar (at the top of the screen), then choose "Open Archive".
To return to your current messages, click File and "Open Archive" again.
You will notice there is a check beside the "Open Archive" option. That means it is a toggle and will take you back to your main mailbox window.
Occasionally go to your Archive window and clean out any unneeded messages you may have.
If you have your e-mail in a folder on your main desktop and archive the items in that folder then GroupWise will create that same folder in the Archive for you. If you prefer you can go out to your Archive and create any folder you may need before Archiving.