Add dialogue:opengroup capability

[namiarj]

Added dialogue:opengroup capability to control who can open a conversation with multiple participants.

[danmarsden]

nice idea - can we also change the form selector in the conversation starting form so that it only allows a single user to be selected rather than allowing them to select lots of participants and then getting an error on save? - it's usually better to prevent them from being able to add multiple users rather than just throwing an error to tell them they are not allowed to.