You ask other player's Sim to move in to your household and become their besties or soulmates depending on your story!
You need to reach level 8 relationship and a free Sim slot before you can ask someone to move in.
Sims that you ask to move in will carry all of their attribute like Traits, Create-a-Sim items, etc. The only downside of this is when you put some expensive stuff to your Sim and someone ask to move-in that Sim, that player can keep the items that your Sim wears!
Firstly, you need to have an empty Sim slot. You should also capable of attending parties so you should be at least Player Level 6.
Reach level 8 of any relationship stories, you can start a rival story and still able to ask them to move in.
This interaction will become available once you've reached level 8 of your relationship.
Once you've met and unlocked the interaction above, tap on it. This notification will appear in your end, tap the "Request to Move-in" to proceed.
The other player will receive this message in their inbox. Keep in mind that you can't send a move-in request when there's a party going on.
Once the other player tap the "Move my Sim" option, his/her Sim will immediately retire and leave an heirloom. The retired Sim will roam around your town and eventually become an elder NPC overtime.
After the other player accepted the request, this message will appear in your inbox. Tap "OK" to receive the new Sim! You'll be directed into "Create-a-Sim" page and edit them to your hearts desire!