Fellow is informal for a member of a social circle. Kleo Ng gives some of the formal meanings of fellow.
Buddy, chum and pal are informal words for friend. All three of these mean friend, but they are all somewhat outdated. More contemporary words in American English include 'bud' (yes, it's a short version of 'buddy'), and 'amigo,' which is borrowed from Spanish. But the most common word is still 'friend.'