Go to the users page, on which you also can search for specific users. Be aware, that the default search only shows "active" users, which is reflected in the page title. This can be changed with the checkbox-filter in the Activity-column.
 
As additional feature, you can follow the link "Show my opponents" on the page-bottom to only show users you have played with or you currently play. There's also a link "Show opponents" on each user info page, that shows all opponents of that particular user.
 
That search page contain more filters above the table, that can place restrictions on the games with your opponents. Following the "Info"-button on a player opens a statistical page about the games between you and the selected opponent.