I've got every team member, even Zaeed, except for one. I don't know who or what it is or where to find this last ally. Is it just a normal part of story progression to eventually find the last team member?
Grunt - Found in the Engineering Deck after saving his clone pod from "The Warlord". You need to go into the Cargo Bay and open up his clone pod, then have enough Paragon/Renegade points to convince him to not kill you, to get him to join your squad.
Legion - Found during a sidequest, (Or was it part of the main quest? I can't remember :|) after rescueing him his deactivated body can be found within the AI Core on the Crew Deck. Activate him and ask him to join your crew, he will accept. Be warned, you'll need a significant amount of Paragon/Renegade points to prevent an "incident" occuring later on, which could result in Legion being killed.