Welcome to the Forum, ACMiku!
The easiest way to have your sim and friends all in the same household would be to create the household in CAS at the same time. In the lower left hand corner, there's a button to add a sim, and you can have as many as 8.
There's not really enough room on most of the lots to have more than one house. You can add other sims just by clicking on an empty lot or house. You'll get the option to go to create a sim, where you can make your friends.
If you really do want everyone to live in the same house, you can then send your first sim to their house to meet them. Once the friendship status is high enough, you get the option "Ask to move in." When that happens you can either move your sim in with them or move all of them in with your sim.