Details for how to recruit - and how to retain the services of - Wrex.
One of the few optional characters in the game, Wrex can be recruited after meeting with the Council for the first time. In the chat with Anderson and Udina after the aforementioned meeting with the Council you can get referred to Barla Von, which is one way to find out about Wrex. Alternatively, you can just head to Chora’s Den to find the krogan threatening one of Fist’s Bouncers.
Whether you talk to Barla Von or witness Wrex making threats, all you need to do is subsequently head to the C-Sec Academy area to find Wrex doubling down on his threats. Approach him and accept him into your squad.
Recruiting Wrex is one thing, keeping him around until the end of the game is another matter entirely. He’s the only character in the game who is both optional, and whose services can later be lost. On Virmire you’ll have to convince wrex to remain loyal to you, which you can do with 8 ranks of either Charm or Intimidate. Failing that, you can complete the assignment Wrex: Family Armor, which will make talking Wrex down much easier.
More information about this choice can be found by following the link - beware of SPOILERS.
You’ll unlock the Krogan Ally achievement for completing five missions and/or assignment with Garrus in your active party.
ME1: Complete 5 missions with the krogan squad member