If there were only 3 things to do when onboarding a new employee in a company using Asana, that would be:
Add them to the relevant teams & projects in Asana. I very often see new employees not being a member of any team or projects and as a result just creating tasks not attached to anything.
Encourage them to disable email notifications, teach them to use the Inbox properly and ask them to fill in their name, role and profile picture.
Make sure they master My Tasks view and check their task list on a regular basis, you could assign a “champion” to them to help!
Anything I forgot that is critical?