This can be done three ways:
Using ldapmodify, create the following ACE string in the domainAccess preference of the icsExtendedDomainPrefs:
Form the ACE by specifying the domain allowed to search this domain, followed by sufficient permissions to allow the search.
Using Delegated Administrator Utility command commadmin domain modify, add ACE strings specifying the domainAccess preference in icsExtendedDomainPrefs attribute.
For example, in a Schema 2 environment, sesta.com allows searches from siroe.com:
commadmin domain modify -D admin -w adminpassword -X hostmachine_1 -d sesta.com -A +icsextendeddomainprefs:"domainAccess=@@d^a^slfrwd^g; @siroe.com^a^lsfrwd^g;anonymous^a^r^g;@^a^s^g"
Using Delegated Administrator Console, when creating or editing an organization's properties, you can add domains to the Allow Invitations From Users in These Organizations list.
This updates the domainAccess preference in the icsExtendedDomainPrefs attribute.
While you can specify the exact permissions given to the domains in the first two methods just listed, the last one, using the Delegated Administrator Console, does not allow the administrator as much control. The list of permissions is preset. The permissions given are: free-busy access, and event scheduling access. The user can't see event details unless the owner of that calendar has set permissions to allow all users to read it.