Go into the main area and find Wrex. He'll be happy to see Shepard and will chat about their past endeavors and how he has united many krogan under Clan Urdnot. Shepard can ask if Wrex will join the squad again, but he will decline. Wrex may also be questioned about Grunt's condition and/or Mordin's old assistant.
After Shepard has spoken with Wrex, the mission is complete.