We belong to both II & RCI because we own other timeshares that are affiliated with each. By contrast, DVC's members are not able to join unless they own some other timeshare. For whatever reason, DVC chose to contract with II as a corporate member rather than permit members to pay for their own individual memberships with II. While this eliminates the annual membership fee and reduces your exchange fee, it also eliminates the option for an individual membership.
If you own another timeshare that is affiliated with II, you may join as an individual. Annual dues are about $75 per year. Members have access to additional rental weeks of "surplus" inventory (weeks that are not likely to fill an exchange request or weeks that were given/sold to II by timeshare developers). These vary from $199-$649 for a week, depending on size and desirability of the unit/resort/location/time. There are also other travel services available, similar to those offered by DVC Member Services (air, car, etc.).
We really like having our exchange company memberships. It's a shame DVC & II don't have a contract to permit it.