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 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.
Update: we're currently running an early alpha 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 email@example.com
Related: How do I update my email address?