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.

The Sims 3
In the Sims 3, you must have somehow of friend or higher and perform friendly socials until "[Friend] thinks [Sim] is being friendly" appears. You can then ask them to move in. Note that if your target has the Commitment Issues trait, it will take longer to invite them. Also, if the target is a single parent, you MUST move in their children as well.