Does anyone have any general tips on the writing process, when the music is a collaborative effort--more than one person writing it?

I've been in a band before, and written bass lines for the other members' melodies. But lately someone has been writing a melody and chord structure, and wants my input to go with theirs. They give me something they've started writing, want me to add to it, and then they may or may not make further changes to it.

Just looking for general advice.