In the post, the studio talks about how it's supporting cross-realm play through three primary features. Players will have the ability to talk to anyone on any realm using the chat system, they'll be able to group up with friends through the party system, and they'll be able to meet random strangers through the group finder.
The cross-realm play does have a big limitation, however. Cross-realm parties will not be able to hang out on the landscape, but can only enter instances, battlegrounds, or arenas, and these parties will not be able to trade items between each other. Players who are not interested in cross-realm play will also have the option to toggle their account to group with only members of their server.