They are both close friends.

Make it simple for yourself; "They are close friends". 'Both' indicates two individuals but for you to use this in a sentence about 'friends' would mean it is already implied you are talking about two people. There is nothing wrong with "They are both close friends". Some examples to consider: They both like the same man (can mean they are attracted to, or are similar to, likely the former) They are both in love with the same man They both love the same man They are both like each other (they are similar to each other) They are both liking the same music (general statement - implying continuous enjoyment of various music) They are both liking the same man (INCORRECT SENTENCE - makes no sense) They are both like the same man (INCORRECT SENTENCE - makes no sense)

Traffic rules are important TO us means the rules are important TO the individuals in the group. Traffic rules are important FOR us means that the rules are important FOR us collectively. Both are correct it just depends on the point you are trying to convey.