The business structure of the ECHL (and for the most part NHL, AHL, CHL, etc.) is the franchise model.
Owner (or group) purchases franchise from league or previous league owner. As part of their ownership, there may be league "dues" paid annually (this might not be "out of pocket" but deducted from central league revenue paid to franchises).
Should an owner/group have a dormant franchise, there is an annual "dormancy" payment that is made to keep the franchise inactive (but valid). If not paid, the franchise is given up (and "returned" to the league who may chose to sell it, or not).
Or do memberships have to be renewed each year? I thought they had to be renewed(just a guess on my part)
They don't have to be renewed each season. Most teams sign a long term deal. Example the Komets signed a 3 year deal when joining the league this off season and Evansville signed a 10 year deal. I would almost imagine that the ECHL has a 3, 5, and 10 year deal set up in advance. Don't know if that is true or not but seems logical.