Currently we have cal.Room as site_model, and we agreed that this is not a good long-term solution. Another aspect is that we need a table which links a given user to one or more sites (N:M). We have the xl.lib.teams plugin but this plugin is for when every user is member of exactly one team. So we cannot use it here. So maybe the best is to create an explicit plugin for implementing the sites_model. Maybe sites.Site and sites.SiteRole.