A Virtual Organization (VO) is a group of grid users with similar interests and requirements who are able to work collaboratively with other members of the group and/or share resources (data, software, expertise, CPU, storage space, etc.) regardless of geographical location.  A list of all registered VOs is available from the CIC Portal.  You will need to be a member of a VO before you are allowed to submit jobs to the grid.  Exceptionally, if an existing VO is not appropriate a new VO can be created.