Scroll to the "Join Available Organizations" section at the bottom of the page.
Select the "Send Email Verification" button and follow the instructions in the email to verify your email address. (Skip this step if you do not see this button).
Once your email is verified, you will see a list of organizations you can join.
Select the Organization you wish to join.
If the above doesn't work, you can ask a Flowdock Organization Administrator to invite you to the Organization.