In Bonfire, you can always find the button to sign into or out of your account in the top-right corner of the screen.
To sign in, click the blue LOG IN / REGISTER button. You can also sign in from the main portal page by clicking the Log in tab:
You can tell that you are signed in because the corner will be replaced with your account's profile picture and name. To log out of your account, click the chevron by your name, then Logout.