The Grand Council creates new members semi-regularly in the spring of each year, calling for self-nominations. You can apply here. The best way to join is to subscribe, keep up-to-date on reading, and respond to the call for self-nominations.
Another method of joining the Grand Council, is to have your Kingdom appoint you a member. Each Kingdom (at least by tradition, if not formal statute) has the right to appoint a member to the GC. If your Kingdom does not have one, or your representative is not active, follow your local Kingdom processes to get an appointment.
If you are Kingdom Appointee formally joining the GC, please notify the following people after you subscribe :