If this is happening for all users in your Apps Domain, have you checked the parent sharing settings?
The issue is probably that you have set all calendars to inherit parent settings from the Admin account. In this case you can change the Admin account settings to allow all users within the domain to share event details with people outside the org.
Here's a link to the help article on how to manage the domain-level settings. Relevant sections below:
In the Org Settings tab, select the organization for which you want to
set calendar sharing options.
Select the top-level organization to enforce the same settings for
your entire organization. You can also specify different settings for
each organizational unit. If you have a lot of organizations, click
the + button to expand your options.
Under Settings in the left column, locally applied means that the
settings are not inherited from the parent organizational unit.
(Settings at the top-level organization are always locally applied.)
Inherited means that the settings are taken from the parent.
In the External Sharing options for primary section, select how you want to
allow users to share their primary calendar outside your domain:
- Only free/busy information (hide event details): Users can allow
people outside your domain to know when they are busy or available,
but event details are hidden. Private addresses are hidden from users'
calendars.
- Share all information, but outsiders cannot change
calendar: Users can share their calendar information with people
outside your domain. This includes guest list, location, and
description. Private addresses are hidden from users' calendars.
- Share all information, and outsiders can change calendars: Users can fully
share their calendar information with people outside your domain. This
includes guest list, location, and description. Private addresses are
displayed.
- Share all information, and allow managing of calendar:
Users can fully share their calendar information with people outside
your domain, and they can grant outsiders permission to manage their
calendars. Private addresses are displayed. In the Internal sharing
options section, select the default internal sharing level for primary
calendars.
NOTE: There are also internal settings options that you can set within the Admin dashboard, so just fiddle around and see what settings work best for your group.
