Repro:
1. Pick Tusk and level up Snowball
2. Create an allied hero (and give them necronomicon, to have non-hero example as well)
3. Position the allied hero (and his necro units) between Tusk and an enemy
4. Cast Snowball on the enemy
5. Cast the Launch Snowball sub-spell to start rolling
6. While the Snowball is rolling, select the allied units and right-click the Snowball in attempt to enter it

Result: Allied units cannot enter an already rolling snowball.

Expected: Since 6.84, it should be possible to enter a rolling Snowball. Tusk himself can pull you in while rolling towards the target, but you cannot enter it by clicking on it. You only can when it is stationary. It should always work.