The group mail service is defined in a Class-of-Service definition called standardGroupMail:
# # Definition for group mail service bundle # dn: cn=standardGroupMail,<ugldapbasedn> changetype: add objectclass: top objectclass: LDAPsubentry objectclass: extensibleObject objectclass: cosSuperDefinition objectclass: cosClassicDefinition cosTemplateDn: o=cosTemplates,<ugldapbasedn> cosSpecifier: inetCos cosAttribute: mailMsgMaxBlocks daServiceType: mail group NOTE: When the Delegated Administrator configuration program installs the standardGroupMail definition in the directory, the variable <ugldapbasedn>, shown above, is replaced by your root suffix (such as o=usergroup).
The daServiceType attribute defines this as a mail service for groups.