The Uncharged ring can be obtained by talking to the Quartermaster in a Clan Citadel. To gain access to him, you must have a sufficiently high rank to enter the keep (inner set of doors).
If you have gathered enough resources, you will get a message to talk to the Quartermaster to claim your ring. He will give an uncharged ring, which can be charged by using it on one of the skill plots. The ring will give bonus experience in that skill. The amount is dependent on your level, and the tier of the plot. The rings are:
When you have earned all of your ring's bonus experience, you will receive a message stating "Your clan ring has used the last of its charge and crumbles to dust." You can receive another ring after the next build cycle.
Note: You may only have one Uncharged ring at a time. If you try to get another from the Quartermaster he will tell you to use up the ring currently in your possession, even if you have gathered enough resources for a new ring.