You ought to be able to perform the same action on a selection of ideas portal users.
Here's a scenario I recently encountered:
I created an ideas portal for someone else's project. In advance of that person announcing the project and calling for feedback, I granted a list of ~100 people access to the portal. However, I didn't want to send the email invite too soon, so I left that option off when I added them to the portal.
Now, many of these users can't access the portal, they can't sign themselves up to it. Right now, I have two options:
For every. individual. user. Click edit, and click the option to resend email invite.
Delete the portal and replace it with a new one (this time don't mess up the import of users!)