If you are unable to complete your bulk use upload you should consider the following:
1. Is your file in the correct format? Check out this article for help on file formatting Adding Multiple users (Part 1)- Introduction and tips
2. Are you missing required information? Are there blank cells where required information should be, for example are there cells missing the users email address
3. Are any date fields in a valid format? For example do you have have any cells which have NULL rather than a valid date?