My sim lived alone and I tried going to Lounges etc to find people playing instruments but it didn't work. I ended up adding a random sim to the household for this achievement, then removing them afterwards.
1. Save your game and go to the Manage Worlds screen
2. Click on your sim's house
3. At the very bottom right of your screen, cick the '...' (More) icon
4. Then click through to 'Manage Household'
5. On the resulting popup, click the Pencil icon to add/remove household members via CAS.
I also found that tutoring the sim for that long caused them to have a really high friendship. In hindsight, I probably could have kept that random sim and after the achievement, used the 'Transfer sims between households' option (from the menu in step 5), to move them to a random unplayed household.