"At night the comrades arrive."
Translation:Noctu contubernales adveniunt.
In this course they seem relatively interchangeable (not sure if they always are however).
Comes (from which comites comes) seems to be more general, a companion, someone one goes with. The word comes from the prefix com- (with) and eo (I go) from what I have seen.
Contubernalis (from which contubernales comes) is more specifically a member of a contubernium, a squad that would share a tent together. The squad would all be comrades, tent companions.