Sharing across all domains hosted on same server is disabled by default, but if all domains on same server are owned by same company, then you can enable this in iRedMail Easy web portal:

- Go to mail server profile page
- Click the "Settings" tab, find section "SOGo Groupware"
- Toggle on option "Allow cross-domain global address book".
- Save your change and click the "Apply Saved Changes" button on top of page.

It's impossible in this case since SOGo doesn't offer a flexible way to query / filter the accounts.

What does this mean?

this just edits a sogo config file

I tried set
SOGoDomainsVisibility = ((domain1.com, domain2.com, domain3.com));

but it does not work. If I'm not mistaken it works only when user domains defined as:

>>>      domains = {
>>>        dom1.de = {
>>>          SOGoMailDomain = dom1.de;
>>>          SOGoUserSources = (
>>>                  {
>>>                    type = sql;
>>>                    id = dom1_dir;
>>>                    DomainFieldName = "domain";
>>>                    viewURL =
>>>    [2]"postgresql://sogo:passw...@mail.lan:5432/mail/sogo_users";
>>>                    SOGoDomainsVisibility = ((dom1.de, dom2.de));
>>>                    canAuthenticate = YES;
>>>                    isAddressBook = YES;
>>>                    userPasswordAlgorithm = ssha;
>>>                  }
>>>          );
>>>        };
>>>        dom2.de = {
>>>          SOGoMailDomain = dom2.de;
>>>          SOGoUserSources = (
>>>                  {
>>>                    type = sql;
>>>                    id = dom2_dir;
>>>                    DomainFieldName = "domain";
>>>                    viewURL =
>>>    [3]"postgresql://sogo:passw...@mail.lan:5432/mail/sogo_users";
>>>                    SOGoDomainsVisibility = ((dom1.de, dom2.de));
>>>                    canAuthenticate = YES;
>>>                    isAddressBook = YES;
>>>                    userPasswordAlgorithm = ssha;
>>>                  }
>>>          );
>>>        };
>>>      };

which is not the case how it's configured on iRedmail install