How can I open access to everyone with an email address from a certain domain as editors?

In a 1.0 wiki, this is a paid feature. When 2.0 goes live, it will be a paid feature there as well, and will have a similar procedure for activating it.

This is the procedure for a 1.0 wiki:

Log in as the Admin
Go to Settings, then Access via email
Type in the email mask in the form "@example.com"

This is what the page says:

"As an example, alice@example.com, bob@example.com, and carol@example.com (plus a hundred other people with @example.com emails) all need to access this wiki. If this wiki's access-by-email setting is @example.com then anyone who can receive an email at example.com would be able to access this wiki. When prompted to log in, users will have the option to enter an email address to which PBwiki will mail a specially encoded URL, and they'll receive Contributor access. Easy!

You need to use '@example.com' rather than 'example.com' so we can make sure 'evilexample.com' doesn't qualify for email access. Note that all visitors to the login page for this wiki will be given the option to use their '@example.com' address for access. Set an empty value to disable this feature."

If you'd like more information on pricing plans, especially those being implemented in 2.0 when it is released, I recommend you email sales @ pbwiki, I think they'll be able to help you most effectively get the features you need for your wiki.