GlassFrog allows you to join one organization per email address. This means that if you want to be a member of two organizations, you will need to use two separate email addresses.
Fortunately, most email providers allow the use of 'aliasing'. For instance, if you use GMail or Google Apps and your address is firstname.lastname@example.org, you could register a second GlassFrog account by using the alias email@example.com. Anything after the + is ignored, and all the emails will arrive at firstname.lastname@example.org.
Also, if you're a Chrome user, there's a Chrome extension called Sessionbox which will allow you to manage multiple logins with the same browser.
See this GMail support article for more information.
Related: How do I update my email address?