The best way to send out bulk password reset is to select all accounts in the "people" section, choose "send email" in the operations section, then craft your own message and past the following as a reset link:
[site:url]/user/password?edit[name]=[user:mail]
You will need to use the token [user:mail] for the "to" field.
The resulting email send out to all selected clients should look something like the following:
Please bear with me while I get my new file sharing system up and running. Here's a password reset link for your new account: https://demo.cpagardens.com//user/password?edit[name]=client@cpagardens.com