If you are using the Basic edition, go to my.riffremote.com to sign up and then sign in. Once you sign in, you will be directed to the Lobby, where you simply enter your first name and click Join Room. If you want to host a meeting, click 'Host a Meeting' in the top bar. From here, enter your name and a meeting title, then click 'Get Invite Link' for a link you can send to others. Then, when you have your link copied, click 'Host Meeting'.
If you are using the Business edition, you have created or been given a custom URL to use. Again, once you sign up and then sign in, you will be directed to the Lobby page. Enter a room name and your first name in the respective fields and click Join Room.