Perhaps this should really be added at the end of this other thread in the support forum; but I intend to make some concrete suggestions, so I'm posting it here instead.
I have to say, I love the way XenForo does conversations ... up until somebody leaves, that is.
The way XenForo behaves when users leave a conversation seems to be based on an assumption that XenForo will be used primarily for social interaction between relative strangers. From that perspective (and given the generally poor etiquette of many internet denizens), it is reasonable to assume that users will want to leave a conversation without signaling their departure.
But as Floris pointed out, this behavior is not necessarily appropriate in a business environment. For conversations between business associates, it may make more sense if all the participants know just who is listening at any given time. Misrepresentation of this information can lead to confusion and hard feelings that will interfere with the successful conclusion of the business at hand.
I don't think it's necessary to eliminate the option of "sneaking out" of a conversation; but I think it should be just that: an option, not the default behavior.
Here's how I'd like to see conversations work:
Starting a conversation should work just as it does now. Thereafter, the system should log changes (or imminent changes) in the number of participants by inserting a message within the conversation itself. For example, when a user is invited to participate in a conversation, the system should alert all participants to this fact by inserting a message that says:
User_A has invited User_C to join the conversation.
Then when the invitation is accepted, the system would insert this message:
User_C has now joined the conversation.
This sort of logging (timestamped, of course) can be useful when a participant wishes to look back over a longish conversation and understand how it evolved over time. It's not exactly necessary, since any of the participants in the conversation can do the same thing manually; but having the system do it automatically would be a nice labor-saving touch.
Now when a person clicks the "Leave conversation" link, he should be offered a choice of two options that are slightly different from the ones currently available in the "Leave Conversation" dialog:
Option 1: The user can choose to "Leave the conversation". In this case the conversation will be removed from the conversation list, and the user will receive no further alerts about it. The system will log the user's departure by inserting a message in the conversation:
User_B has left the conversation
The number of participants in the conversation sidebar will decrement, and the avatar and/or name of the user who left will be color-coded or something similar (as Treasurer suggested) to indicate that the user is no longer actively participating. Current conversation participants will be able to invite the user to rejoin the conversation, if they so desire.
Option 2: The user can choose to "Ignore the conversation". In this case the conversation will be removed from the conversation list, and the user will receive no further alerts about it, just as with Option 1. But the system will not log the user's departure. The number of participants will not decrement, and the user's avatar and name will remain displayed as if the user is still actively participating. Current conversation participants will not be able to invite the user to rejoin the conversation.
You'll notice that I've more or less revised the current first choice, which allows the user to remove the conversation from his list but still receive alerts when a new message is posted. I can't really see the point of such an option, since it has no real effect other than removing it temporarily from the conversation list. (If someone can explain any other benefit of this choice, I'd love to hear it.)
As you can see, Option 2 is essentially the same as the current second choice. By revising Option 1 (and adding the logging behavior), I believe XenForo could become a more reliable platform for business conversations. This would hopefully extend its appeal in the business market, leading to increased sales.