[blind-eddie]

Go to you Admincp/Usergroups/Usergroup Manager, click on desired Usergroup.
In the very first section, you will see the line that reads,

Password Expiry
(If you specify a number of days here, users will be required to change their password when this amount of time elapsed since they last changed)

Change that number to a very low number, If you have one set there.
Send a mass email to all members informing them of site related items.
Not the issue at hand.
This will get them to your site & see the pop up that tells them that their password is now expired.

Monitor your site a day or so, then up the number you set in the Password Expiry to what ever you wish.

Note, this will need to be done to all usergroups with site privileges.