Support multiple teams for a single email

HipChat does not currently allow one email to be on multiple teams. This is a problem where collaboration is required for multiple organizations, where the team members themselves have no control over the email assignments. For example, I cannot invite customers to collaborate in a non-guest mode and join my team, if their company also uses HipChat for other purposes. Creating a separate email for my team would be a policy issue for them as would creating a separate email for me to join their team, since I would not be considered part of their organization. This seems like a significant deficiency to the whole collaboration concept and I am surprised it is not part of base capabilities.