The usernames you see in "Who's Online" at a board are Runboard members but they are not necessarily members of that board.
Types of Membership
Everyone starts as a basic member, which gives them access to most Runboard functions. They can post, create a board of their own, have access to Runboard personal messaging, add their own avatars/signatures etc. And unless they are visiting a Premium Board, they will see advertisements (but no pop ups).
For a small annual fee, you can upgrade to become a Premium Member. That enhances existing features (eg. bigger post size, bigger inbox) and provides extra features, such as invisibility, new message notification and no advertisements.
The differences between basic and Premium memberships are listed ]HERE
To upgrade your user account to Premium for one year (USD 10, no recurring billing) please go to your ]Control Panel and click the "Upgrade my account" button.