If you're developing a friendship with a sim, you will get additional contact bonuses every level. They will appear in the messages. The same happens when you're developing stories.
Each sim controlled by other players can be gifted a sticker. They change every week. Depending on the amount of stickers a player gets, they will receive an amount of points and fashion gems. Giving stickers is one of the daily activities.
You can visit another sim's house at any time. Just click the sim and then the icon highlighted on the screen above.
You'll be able to select the "visit house" button and you'll be moved to the house. Unfortunately, you cannot do there anything - visiting is about everything you can do. If, however, one of your friends is making a party, you will be notified. On such occasions, you can interact with the house in the regular manner.