The United Federation is a Ship Achievement for the Kestrel Cruiser.
"Have six unique aliens on the Kestrel Cruiser simultaneously."
Humans do count towards this achievement. The 9th crew member counts too (when you get an extra person from an event and have to dismiss someone).
Including humans, there are eight different species in the game. Many of the species can be found fairly easily either through random events or hired at stores, but the Slug, Lanius, and Crystal are more difficult. The most reliable way to find a Slug is in a Slug Home Nebula sector where they should be available to hire as crew in a store there or through select events like Slocknog. Same with Lanius, except they are found in Abandoned Sectors, and there is an event where the player can acquire a Lanius. To find a Crystal crew member, the player must receive the stasis pod from the Empty Ship in Asteroids quest, and then have the pod activated in the Zoltan Research Facility quest.
With the initial three human crew members in the Kestrel, there is exactly enough space to take on one alien from each of the other five non-Crystal races and get this achievement. If another human or a duplicate non-human alien were hired, one of them would have to be dismissed/killed at some point to earn this achievement.
This achievement is somewhat easier to complete using the Red-Tail (Kestrel B), as the starting crew includes three of the six required races. It is also easier to complete using the Kestrel C as the starting crew already contains one of the three rare races.