Game guide:Easy friends

By cheating
By using boolprop testingcheatsenabled true cheat, you can simply drag the daily and lifetime relationship bars into left or right. BOTH Sims have to have relevant relationship and interact at least once after relationship drag in order to make friends. Note that this will not change reputation as reputation requires interaction, not friends.

Nightlife

 * After a good date or outing, a contact (A.K.A. friend of friend) might make a call and ask for an outing. They make instant friendship but only for a while until the contact effect wears off. Sims should befriend them in the meantime.

Open For Business

 * Any Perk in Connections Business Perks will help relationship boost for Sims.

Seasons

 * Certain seasons affect relationship boost. Spring boosts romantic interactions, Summer boosts friendly interactions, and Winter boosts family interactions.
 * Wishing "Friends" in Wishing Well will spawn 3 instant best friends at first interaction. On the other hand, the bad outcome will spawn 3 instant enemies.

Bon Voyage

 * Bigfoot will make fast friendship for Children and Dogs, and find it harder for Cats.
 * Using Voodoo... Friendship interaction will make friendship boost for the target.

FreeTime

 * The ultimate Aspiration Benefit for Popularity, "Fast Friends" will make quick boost for friendly interactions to any Sim.

Apartment Life

 * Doing correct gestures and telling correct stories to a corresponding Social Group Townie will make a relationship boost.
 * High reputation gives a daily and lifetime relationship boost for newly met Sims.

The Sims 3
If you need fast friends, there are heaps of ways to get them. One of the ways I use is buying the Office Hero lifetime reward and building up Charisma skill. Then, you will become friends with your co-workers very quickly, if you pick the socialization tone. This varies from career to career. Examples are "Chat at water cooler," "Discuss News with Co-Workers," etc. An easy way to get friends, with no effort on your part.

But the next way is one of my favorites. Buy the Legendary Host lifetime reward, and meet some Sims. Then throw a party, inviting them. With Legendary Host, they will always come to the party, even if they hate you. Just invite the same Sims for a few consecutive parties, and watch the relationship values soar. Even if you mistreat them, they will still get a gain.

Playing a Guitar in public with a high skill will increase your relationship with anyone who watches, also an easy way to earn money.

So, there we have it. Three easy ways to get friends, which are guaranteed not to mess up your game.

With Cheats (Sims 3)
Press Ctrl+Shift+C and type in testingcheatsenabled true. Then, on the friends tab, drag the friendship bar up or down. This can also be used for needs.