This forum website consists of cookies concept. So when a user logs in with their username and password (by checking/ticking the 'Remember me' button) the first, a web cookie gets created for the site with that username in that computer. So from the next time whenever you open the site in that computer, by default the site will have that particular username logged in by default.
How do you login with another username?
Logout from the earlier username. When you login with your username/password, remember to tick the 'Remember me' button. So that it has a cookie created for your name too. When multiple users access the site from the same computer, they need to follow these steps, if cookies is enabled in their computer.
Note: You can also disable cookies from your Internet explorer browser options, so that there will be not be any default logging in happening when you open the site.