To use LCG resources you must be affiliated with one of the following Virtual Organizations (VO). You will be required to select your VO when you register and the information you supply will be forwarded to the VO administration and resource prividers for validation before you complete the registration process.

Currently you can only be a member of one VO at a time.
If you decide to change the VO you belong to, please send an email request from the registered address to lcg.registrar@cern.ch. You will be asked to confirm your request by the LCG Registrar. An email notification will inform you when you have been removed from the registration database and your previous VO.
After this you should submit a new registration form selecting your new VO.

Virtual Organization (VO): "abstract entity grouping Users, Institutions and Resources (if any) in the same administrative domain" - I. Foster, C. Kesselman and S. Tuecke, The Anatomy of the Grid, International Journal of High performance Computing Applications, 15, 3 (2001)