If you're unfamiliar with rundisney registration, be sure to join (if you haven't already) the Dark Side thread on the rundisney forum here! There will be lots of helpful advice there.
You're just supposed to book your family.
While it is actually possible to register others*, there is the possibility of rundisney looking into the registrations and cancelling those who aren't members/same-house-family.
*and actually since each adult registering has to have their own active.com account, and there are legal things to read and click that you agree to and to initial during the registration process, legally you shouldn't register another adult anyway. I'll register my husband, but I'm not comfy (anymore) clicking to say "yes I read and agree" for an adult other than DH.