Grayskull was the name of Mara's unit that the First Ones had sent to settle on Etheria, and later was used as the name for the Rebel Squadron that she formed to prevent the First Ones from activating the Heart of Etheria. They failed to activate the failsafe, which left as the only option for Mara to open the portal to protect the universe. It appears that Mara was the only surviving member, just barely making it to her ship and transporting Etheria through the portal to Despondos. They left a recorded message with instructions on how to locate the failsafe to deactivate the Heart.
The named members of Grayskull are Mara and Serenia.
Serenia, in the recorded message in Return to the Fright Zone: "This is our last transmission. If anyone's hearing this, you need to listen. There's a fail-safe for the Heart of Etheria. It will allow you to destroy the Heart and end this for good. It's at the Arxia outpost, but we lost touch with them a week ago. If there's anyone left out there, you need to find it. Mara bought us time, but it's too late for us. This is Rebel Squadron Grayskull. And for the last time, we're signing off."
According to ND Stevenson, "For The Honor Of Grayskull" was the battle cry of Mara's squadron, Grayskull. It was also the password for the sword's activation, as seen in "The Sword: Part 1".
"For the honor of Grayskull!" is later what Adora shouts when transforming into She-Ra.