@TTKINCORPORATED
If all your slots are full, you have to retire a sim to move another sim in. You can only have four active sims. Also is the sim your trying to move in an npc sim or does another player own the sim? If another player owns the sim, you need the player's permission to have the sim move in your game. When you ask another player for their sim , that player looses their sim from their game to add to your game should they permit you to move their sim in your game. It is best to stay away from getting your sims involved with other player's sims unless you discuss before hand and they agree to give their sim away. If they are not willing to give you their sim, you can not move their sim in your game.