Interesting I'd not realised 'Everyone' was a role as such, it's there to support one specific field type which is the hyperlink field, you should be able to add this with the 'Everyone' role selected under the 'More fields' section.

The idea is that you can force all participant in an agreement (everyone) to click the hyperlink.

The signature and signer info fields, always need to be assigned to a specific participant. So if you are sending the agreement to more than one participant, then add 2 or more signature fields, changing the participant before you add the new signature fields (though you can edit the field properties after placing them as well, bu double clicking the placed field)