Creatures technically always provide at least half cover:
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend (BD&D p74).
So basically, if you're shooting past someone (or even in melee on the other side of a creature), the attack is made against half cover.
I say technically, because my guess is that most DMs don't remember this and so it's left largely unenforced, largely on the ally side. I personally rationalize this by figuring that characters who are used to fighting together are going to stay out of each other's way. However, if you're looking for the by the book answer, all creatures provide half cover.