GlassFrog only 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 email@example.com, you could register a second GlassFrog account by using the alias firstname.lastname@example.org. Anything after the + is ignored, and all the emails will arrive at email@example.com.
Update: We're currently running a beta testing of multi-support org that would allow you to manage multiple organizations from one single account. If you are interested in participating, please contact customer service at firstname.lastname@example.org.
Related: How do I update my email address?