You can unlock this during the mandatory Ancient Forest expedition, listed under the heading Rebuild the Ancient Forest Camp in the walkthrough. During this expedition, you can find the Lynian Researcher, who will ask you to track down some Grimalkynes. Find their doodles, then follow the Scoutflies to reach their lair, where, after a chat, one of the Grimalkynes will join you. An event similar to this can be found on every map, and furthermore, you can usually find a trio of Palicos chilling about somewhere on the map, one of which will join you if you bring your own Palico to them. As soon as any one of these Tailraiders joins you, you'll get this trophy/achievement.