User A starts a private conversation with User B
User B wants to add User C to the conversation but has no idea how
User B should ask User A to add User C to conversation but they don't understand this is an option
Ideally: User B sees a link in the conversation to "Request Invite". When User B click this an overlay appears:
they can now request:
"Ask User A to add User C to this conversation"
User A gets an alert "User B wants to Add User C to the conversation, do you accept?"
User A clicks Accept.
User C in now part of the conversation.