When you first open FM Chat you'll be asked to sign in. Each FM Chat user needs an account in FM Chat and creating this account can be started right from sign-in: just click "create account".
Once you have an account, you can check "Remember Me" so you won't have to sign in on that machine / device again.
Clicking "My Profile" on the welcome screen will take you to the Users layout where you can manage your profile. Here your can upload an avatar other FM Chat users will see when you're online. You can also change the name users see you as in Chat, and change the theme in which you'll view chats. Play around with the different themes until you find one you like. Note that you can only pick your own theme in Pro; on FileMaker Go, we choose the FM Go theme for you.
Editing your profile will also let you change your notification settings.
Clicking "Admin Setup" on the welcome screen takes you to a list view of users where you can create and edit user profiles, change user's avatars, and edit their notification settings. Needless to say, you'll want to restrict access to this button to admin users.