If I already own a mybalsamiq site and have a team on it, how do I open a different one for an entirely different company, and entirely different group of users?
Balsamiq support answered my question:
Click on SETTINGS in the menu. On the left side you have a section of settings called Site Settings. The last one is ADD A NEW SITE.
Simple as that. For some reason this is not clearly written in the documentation and I couldn't find the answer on the web.
The fact that I missed the option when looking for it is either due to Banner Blindness or the fact that it seems like this section is for "THIS SITE" (after all it's "site info" and not Manage Sites...