Within Teamwork Projects, if you have a user who is unable to log in to their account, you can send them a new invite where they will then be able to create a new password.
To do this, first go to the People area of your site.
In the top right corner of the People list, click the options (three dots) button, and select Send Invites from the dropdown menu.
In the Send Invites modal, select the Resend Invites tab, where you can choose from a list or search for the user's name.
Tick the box beside the user's name and click Send email invite now.
Once the invite has been sent, the user will receive a link where they can set/reset the password they wish to use for accessing their your Teamwork site.
For more information on resetting your passwords: Changing your password