I don't get it. We use Slack and that works for us. We don't IM w/ customers. When we want a mentor or vendor or someone to participate, we just invite them as a single-room guest to our main room. What's the point?
Your solution is pretty good, but it has some drawbacks:
1. when you kick the mentor or vendor out of your main room, they no longer have a record of what happened. Since Sameroom replicates messages across channels, everyone has a copy (like with email)
2. If your mentor or vendor wants to involve someone else from their side, it's not easy (they have to ask you to send an invite). With Sameroom they would control access on their end of the discussion.
3. Your invitee is forced to use Slack, which isn't ideal if they normally use another tool, say HipChat. Even if they already use Slack, they have to switch teams, which... well, it doesn't seem to scale very well.
