Note: you can only create client accounts, not any more administrators. Form Tools allows for a single administrator account which is created during the installation phase.
Form Tools 2.0.3 introduced a number of security features to force client passwords to meet certain criteria: length and content. When creating new accounts, those criteria are defined on the Settings » Accounts tab. Read that page for a description of the various fields. If you have any of the settings enabled concerning the client account passwords, you should see a message on the screen describing the required criteria. An example of this is shown on the screenshot on this page.
For older versions of Form Tools, the security settings are not there: don't worry about them!