2 problems

You will need to go to the Campaign page for your game to see the full player list. There is a light grey section on the right side of the page that will list the first 20ish players in your game. At the bottom of this list, there is a "Show More..." link. Clicking on that will expand out your player list so you can see everyone currently invited to your campaign and whether they're online or not. Can you get me a screen shot of the white box you're seeing? Is this when you're actually playing your game or somewhere else on the website?

Can you tell me the name of the campaign you're experience this, Yanojr?

Thanks, Yanojr. I was able to do a little digging. That little white box is actually a blank card deck that is currently set to "Show". If you go to the Decks and Tables tab on the sidebar you'll see a "New Deck" with a button that says "Hide". Click on the hide button and that will make that weird grey box go away from your tabletop. At some point during your campaign, you must have created that deck by mistake, since that Add button is in the relative same place where other add buttons are on the other tabs. As for keeping track of people online. The number of players you have in a campaign is far beyond what we can reliably handle in a Roll20 game currently. My recommendation is to have the Campaign Page open and look at your player list. Any player with the green icon over them indicates that they are presently logged into the campaign.