It means: Can we become friends?It is a request.Or we can say"jiao ge peng you ba".It's a proposal.

Could we be friends?

能叫个朋友吗？( Could you bring me a friend of yours?) or 能交个朋友吗？( Could you be my buddy friend or sweetheart friend?) I think what you are talking about is the last.

someday you meet a person (maybe you know him or her by your friends,)and you talk a lot,then you find he or she is a good person for you,and you want him or her to be your friend.

it means "can you make friends with me？”