Tweetdeck has adopted the Twitter lists approach.
Adding a user to a list
Move your mouse over the icon/avatar of the user, and select the bottom right icon:
In the User submenu that pops up, select Add to group/list
(A group is the old Tweetdeck system of collating users before Twitter introduced lists. Unless you’ve been using Tweetdeck before you don’t need to worry about that.)
In the pop-up window that appears, select the twitter lists you’ve created to add that user to:
Alternatively you can create a new list by clicking on the Create a new list for … at the bottom of the window:
Putting twitterers into lists allows you to give some order to your list of twitterers you’re following, to monitor specific stories and to reduce the likelihood of missing important stuff.
Viewing lists
To view a list in Tweetdeck, click on the add column + icon at the top of the window (next to the yellow compose button) and select the list from the Groups/Lists tab.
The column will load at the far right hand end of the existing columns.
[...] Tweetdeck: Groups [...]