Since Lycan deals a lot of damage but lacks disable to stick to his targets, his best partners are those that provide disables. Moreover, the hero fits lineups with other pushing heroes, especially with multiple units.
Lycan is picked in team strategies for two main reasons. First, the hero is a great Roshan killer (you can even solo Roshan very early on once you get Vladmir’s or Helm of the Dominator), which grants Lycan and his team a big advantage. Second, Lycan farms very fast and can use the accrued items to push extremely fast with his summons. In that sense, the hero is used mainly as a pushing and split-pushing carry.
The fact that Lycan farms fast and is mobile and survivable makes it very hard for the enemy team to control his item development, which makes him a reliable carry. He is hard to gank and he can even farm multiple locations at once.