Since phpBB 3.2.8, the group rank is displayed in the memberlist (if the group has a defined special rank).

It would be a good idea to add "Rank:" or, preferably, "Group rank:", just like when you are going to warn an user (see attached screenshots). It would be more explicit. Also, the image rank is above the rank itself in the memberlist (moreover, memberlist.php adds a <br /> after the image rank, and there is no <br /> between that element and the group avatar), whereas it's under the rank in the member profile or in the mini-profile in viewtopic.