Currently, iBoske doesn't allow more than one person to work on the same decision tree, but there is a workaround:

You must create the decision tree/s whose edition you want to share using an account which you can share.

To do that, create that iBoske account with an email, not using any of your social network logins (don't use your Google, Twitter, Facebook now LinkedIn accounts).

Later, you can share the email address and password used on iBoske with your collaborators, so you all have access to edit that decision tree/s.