I'm working on a messaging app where it is similar to Facebook messaging between friends, like Facebook any number of friends can be involved in the chat. Unlike Facebook (I believe?) users can add in other friends at any time during the chat to join the conversation. What I would like to know is should these newly added users be able to see all previous messages in that conversation?
When it would be good:
Say User #1 and User #2 just spent hours discussing and figuring out plans for a trip, then User #3 decides they want to come too. User #1 invites User #3 and now they can just read through all the plans without have to go over it all again.
When it would be bad:
Two weeks before aforementioned conversation User #1 and User #2 were badmouthing User #3 behind their back. User #1 had forgotten about this before inviting User #3. Now User #3 scrolls up and reads all of the things said about them and "friendships" are ruined.
Note: these examples are just given for context I'm sure there are plenty of other pros/cons to consider, like if bank account details were previously shared it could be more nefarious than simple gossip.
EDIT: One other important distinction from Facebook is the chat instances only span the duration of that conversation history. Think of it like little chatroom hangout where User #1 starts a conversation with User #2 called "Trip discussion" and do the aforementioned scenario, the history would be limited to that thread.