This achievement is obtained by completing Gregg's friendship quest, called "Legends".
In order to obtain this quest, you need to choose hanging out with Gregg in at least 2 of the 3 possible choice moments, on days 4, 5 and 6. Gregg is usually working at the Snack Falcon, next to the Dinner.
When Day 10 comes, you'll only be able to hang out with Gregg. Once you complete the scene, the achievement will unlock.
There's another achievement related to hanging out with Bea, that will need another playthrough.