仲間：なかま： friendship, as in "This event affected our friendship." "Our friendship grows by the week."
But, it also means 'in the same group' (snake is `nakama' with iguana because they are both reptiles. Everyone at the bank is 'nakama' regardless of if they like each other.)
仲良し：なかよし： a good friend.
友達：ともだち： friend (casual situation)
友人：ゆうじん： friend (slightly less casual)