When you adjust your “Close Friends” list, you’ll see an autogenerated “Suggestions” section. It’s basically a selection of people that Instagram believes you might want to share more private posts. While it’s not entirely clear how the app makes this selection, it’s most likely that these are accounts you engage with the most or users you chat with often. If you see a user you want to give viewing access to more personal posts, just tap the “Add” button next to their username. This action will automatically categorize them as your Instagram close friend.