In The Sims Mobile you cannot change your home. The sims can only inhabit the original house. You can, however, move out those sims, which you're not using. That cannot be done with the sims that you can still play - in their case, retirement is the only possibility of getting rid of them.
If you wish another sim to live with your, you just need to invite them for sharing the flat. This is possible only at relationships level 8. You can also have wedding, and the other sim will automatically become a family member. Remember that you need free space in order to do that.