The collaboration API allows you to share a file with another person, regardless of whether that person currently has a box account. The API is setup in such a way that you can detect when a collaboration is accepted by two means:
1) You can add a webhook to the folder containing the file you shared. When an existing user accepts the collaboration that webhook will be called. Tested and verified.
2) You can longpoll your user events and be notified when an existing user accepts the collaboration.
Both of these methods of obtaining notification are broken, however, if you share a file with someone who has yet to create their box account. The creation of the collaboration returns the collaboration to you with a status "pending" and the invite_email of the person you invited to box. When the invitee accepts his invitation and creates his box account, the shared file(s) are there, waiting for him, and the collaborations change out of their pending status.
What DOES NOT happen, however, is a notification event to the account that created the collaborations. Those notification methods work in every other scenario except new account creation.
This appears to be a bug, leaving the only possible work around as repetitive calls to the API to check collaboration status (pull instead of push). Not a desirable solution for anyone.