Game guide:Asking Sims to move in

It is possible to get a Sim to move into a household without going through a marriage.

The Sims
In The Sims, the "Move In" proposition is only available if both adult Sims are of the same gender. Whether it will be accepted accept depends on the relationship between the two Sims, the quality of the asking Sim's house, and the mood of the Sim being asked. Unlike marriage, it can be accepted if the two Sims are good friends.

There are also hacked objects which allow a Sim to be moved into a household.

The Sims 2
In The Sims 2, the "Move In" proposition is available regardless of the genders of the Sims involved. For the proposal to be accepted, the two Sims must have a high lifetime and daily relationship, preferably higher than that needed for them to be best friends. If the Sim being moved in is part of a playable household, the player will be asked which (if any) other household members should also move in.

Also, if any expansion pack or stuff pack is installed, and the Sim is living on a residential lot, he/she can be moved out and moved into the desired household

The Sims 3
In The Sims 3, two Sims must have a relationship of friend or higher for one to ask the other to move in. Have the Sim you want to ask perform friendly socials until "[Friend] thinks [Sim] is being friendly" appears. The target can then be asked to move in. Note that if the target has the Commitment Issues trait, it will take longer to be able to invite them. Also, if the target is a single parent, their children must be moved in as well.

To move a Sim in without hassle, use "testingcheatsenabled true". If Late Night is installed, right-click on the Sim and choose "Add To Active Family". Otherwise, drag the relationship bar to full and do friendly socials until the "[Friend] thinks [Sim] is being friendly" message pops up, then ask the Sim to move in.

Also, if the Sim lives in a household, he/she can be moved out and moved into the desired household.