First you have to have at least 20 points. As long as you do you can sign up for any future run. Do this buy going to the Calendar and selecting the date you wish to sponsor, on the next screen hit the Reply button, make a statment that you wish to sponsor the run.
Please prior the the run that preceeds the run you want to sponsor. I will not accept sponsorship if you do not sign up prior to the run in advance to avoid spam of tells from people that want to sponsor the same run.
If in the case multiple people sign up for the same zone, leaders will decide who get to sponsor and the person that does not will get priorty over others who have sponsored more recently. Points may play a small factor in the determination of who sponsors also.
Any question please feel free to /tell Albak. Thanks