Multiple user groups for each user

I have a system I need to develop whereby there could be hundreds of sets of products each refined to a user group. The idea being that only users with an account can buy products and their account can be linked to multiple groups in order to only allow them access to those sets of products. I am unsure if this is currently possible or if it is as simple as just having hundreds of user groups and then assigning each user to the relevant groups.

Having looked again the system only allows each user to access 1 user group. Does anyone know a way of allowing each user to have multiple-user groups so I can then assigne each user group the relevant products? My other option is to maybe use multi-store and give each user access to the relevant stores...?