Besides the obvious -- seducing someone you've invited over, or are living with -- there are additional situations built into KinkyWorld.
Note that your sim is likely to refuse the invitiation unless at least a bit horny.
Pizza delivery - order a pizza, and the delivery guy may ask for a little reward.
Repairman - Use the "Call for Repairs ($50)" option on a broken object (toilet, TV, sink, shower ...). If aroused, your sim may flash the repair guy, then be asked for a little woohoo.
Note: For me, this situation breaks a lot, e.g. if a guest arrives around the same time as the repair guy. I found it best to save before calling the repair service.
Tom Peeping: If your sim is woohooing alone (masturbating), and notices him jerking off, they may invite him to join.
Note: I've had this situation fail as Tom thought he hadn't been invited in yet, and couldn't path to the bed. Consider masturbating outside if you want Tom to join.