When creating or editing the settings on a group type, members of groups of this type must be allowed to send email.

When creating a new group type, these settings are found on page two of the new group type wizard, known as the Rights page of the wizard.

In the "Group members can..." section, "Email the group" should be checked.

You can check existing Group Types by selecting Groups from the menu then Group Types in the Administration section. Click on the group type that you want to view the settings for.

In the Leader/Member Admin Rights section "Send Email" should be checked in the "Members can..." column.