Which is preferable? Keeping an active list changing (on a grid) that is always visible or to have a button that pulls the list up?
Depends how much your game depends on knowing that information. For instance with a chat program, who is available in the chat is pretty important. But for a lot of games, it may be you only need to know who's available in your guild, or in fact, no at all as you see them on the map.

Question is, what are you personally trying to achieve here?
In response to Stephen001
Well when I make games I like to let people know if other people are online without using tabs. Sooo I usually use grids to display names or I can output their names in a browser.

This game itself, I'm still playing with the idea of using a grid to display players ready to begin, but as for overall players online... no clue.
In response to Lugia319
People logging in having notification in a chat area is quite normal, as it introduces them to everyone and you get the world chat side of it there, builds a community etc. But it's kind of secondary to the game itself, so probably not worth the screen-space to have a permanent grid in your UI for it, unless who is online is critically important (like the chat program example).
In response to Stephen001
Most games take advantage of grids to display player information.

I think as long as it's out of sight and doesn't bother the player, really any method would work.