There is a known issue when importing the users from 3.7 - the thing is SF 4.0 requires a valid unique email for each user, while in SF 3.7 this wasn't the case. So when you import more that one user with the same email (which is empty string in this scenario) you get an exception and a user isn't created. This must be addressed in the next checking of the migration tool.
Also, for now it is recommended that you try and migrate the modules one by one - e.g. select just the users, hit migrate, then select just the libraries, hit migrate, etc.
the Telerik team