This may be the last out of six "Complete 5 missions with X squad member" achievements you'll get as it's related to your missions with Liara T'Soni (if we assume you recruit someone and finish missions with her/him first). But if you want to speed up the process, finish only the main quests on the Citadel and as soon as you are able to travel, go to planet Therum located in the Artemis Tau cluster / Knossos system. Liara is unlocked as a party member after the mission ends, so follow-up this with 5 missions with her in the team.
Assignments counts as missions.