Whenever you want to invite someone who is with a bunch of friends that person has either decide to "leave behind" everyone by accepting the invite, or to join and then asking people to join on them afterwards.

It would be cool if you can send a friend a "invite portal" that they can activate in their instance to get them and everybody around them to the new instance.

could work like this:

[Click to join] [click to spawn a invite portal]

clicking the invite portal will launch a portal leading to the specific instance the invite originated from.