Repro:
1. Pick Tusk and level up Snowball
2. Create an allied Lycan or Warlock (or any hero with summoning spell) and level up their summoning spell (or give them a necronomicon, or HotD and dominate a creep)
3. Summon the summons
4. Cast Snowball on an enemy
5. Select the summons and right-click the allied Snowball in attempt to enter it
6. Now as comparison, select Tusk and right-click the summons in attempt to pull them in

Result: When Tusk right-clicks on allied non-hero units, they do get put inside the Snowball. However, the very same units cannot enter the Snowball themselves by right-clicking the ball.

Expected: Just like allied heroes, allied non-hero units should be able to enter the Snowball by right-clicking it.