Patch 2.4 will remove the restriction for Ritual of Summoning that prevents you from summoning players into instances. Provided the player meets the instance requirements, you'll be able to summon them in from anywhere in the world.

Q u o t e:


And if they don't meet the requirements, does it follow the old rules, or are they not summonable at all?

You couldn't summon them if they didn't meet requirements such as level or attunement.