You can't, only Moderators, Category Owners, Group Owners, and Group Admins can use the message feature. Normal users can not use message features. Would be too easy of course to spam an entire group of users.

GJ 1.8 and 2.0 are nothing alike, we did not develop 1.8 and 2.x is developed entirely from the ground up. The message feature is limited to select users of the group, this is to prevent spam. Allowing any user who joins to message everyone in the group would be an absolute disaster. GJ is of course open source and you're welcome to change the authorization check to allow users to message.

You've need to modify the email function for the respective location (overview, category, or group) in component.cbgroupjive.php. Adjusting the authorization check as necessary. Once done you'll need to edit the respective template file to enable the menu link (xyz_pane.php).

Even in controlled environment it is not recommended due to spam potential. As a single users login being compromised could result in thousands of emails sending from your site. Is less likely for an owner or admin of sorts to be compromised compared to entire userbase.