Thanks for the invite!
Firebase and Slack API seem to be good options. However, personally I would go with a manual implementation. If you have time, resources and want to have 100% control over everything I would suggest implementing something manually. Otherwise, Firebase can give you a great head start and you'll be up and running sooner as the realtime part has been taken care of by Firebase. You should also note that while getting started with Firebase is free, you need to pay once your app scales.
Unfortunately, I have never used Podio. So, I won't be able to comment on that.
So, my choices are : Manual Implementation and Firebase.