Quick and easy method. Took me about 5 minutes.
1: Check lots to find a ghost or get one to move into your lot.
2: Select lot with ghost and click manage household.
3: Edit ghost in create a sim. If there are non ghosts in your household, move them out first or delete them in create a sim.
4: Save ghost to your library.
5: Create new sim from your library.
6: Place ghost into your household.
7: Repeat steps 5 & 6 a further 6 times (so you have 8 ghost sims in total).
8: Make sure each ghosts sims relationship is a family member, so not a roommate. (I'm not 100% sure if this step matters but it does say 'family' in the achievement so I did it anyway, only takes a few seconds.
Not needed, confirmed working if they're just roommates. Thanks, risseless.
9: Once you have your 8 ghost sims, click on play. Select the household you edited and once you've clicked play again, the achievment should pop just as it's done loading.