By default, users can search only within their domain for users and groups to invite to events. Cross domain searches, however, allow users in one domain to search for users and groups in other domains, as long as these requirements are met:
Each domain can specify an access control list (ACL) in the domainAccess property of the icsExtendedDomainPrefs attribute that grants or denies cross domain searches from other domains. Thus, a domain can allow or disallow either specific domains, or all domains, from searching it.
Each domain can specify the external domains its users can search. The icsDomainNames LDAP attribute specifies the external domains that a domain’s users can search when looking for users and groups (as long as the ACL for the external domain allows the search).
For example, if icsDomainNames for the various.org domain lists sesta.com and siroe.com, users in various.org can perform cross domain searches in sesta.com and siroe.com. For a description of icsDomainNames, see LDAP Attributes and Property Names.
For instructions on how to enable cross domain searches, see Enabling Cross Domain Searches.