NOTE: This only applies to the browser-based version of Buzzy. The Android and iOS apps for mobile devices use passwordless login tokens.
Then click the button that is relevant to your email address - for instance, if you want to sign up or log in with your Facebook credentials, click the 'Sign up with Facebook' button. You will then be taken to a screen verifying your choice (eg making sure you want to proceed with you Facebook account - this is controlled by the relevant service ie Facebook in this case, or Google if you chose that option). Proceed as prompted, and you'll be logged in to Buzzy with those credentials.
IMPORTANT NOTE: To access a Buzz you have been invited into, you must use the same email address the Buzz Owner has invited you in with. Buzzy uses your email address to identify you and connect you to your relevant Buzzes.