I can already see the comments: Why are you so cheap? You Open Source addict! Use Google!
But it's too late. I ran into their trap as I wanted to be modern with my students.
So, what's this all about? I asked my former students who are now working in the development world what they are using for managing their team communication. And about 90% recommended slack. So, I took a look at it, it seemed most functions where free, it was easy to use and it seemed what I needed to manage my class. Therefore, I gave it a try.
It worked very well in the beginning, but after about a third of the class it told me that I wasn't allowed to check the history anymore if I wasn't willing to start paying. That was particularly annoying as I needed some data to do some grading and now had to dig these up elsewhere. As the payment for an adjunct professor in the US is extremely low, I didn't want to shell out this money as I thought I was doing a favor to slack introducing it to my students. After my class finished, I was hoping that I could at least export the information and then parse it myself. It turns out that slack only export public channels. Private channels I created will not be exported so its information is lost to me forever. Very annoying.
So, if you run a non-profit, educational, low-budget, or academic project: DO NOT USE SLACK!
You will lose your data and records if you are not willing to pay and you will only realize this when it is basically too late and your are trapped. It is very sad to let these communications of my students and their work progress go, but at least I have learned to be more careful.
Better host your own solution - maybe run your own IRC-server, hangouts, maybe Gitter is a bit more fair regarding history, but slack, no.
Congratulations, slack, you tricked me! At least state this clearly when you open up a community: "Hey there, soon you will not be able to access any history if you don't start paying right away. This free thing is just a very crippled version, you should not even consider using in a productive way."
I do understand and encourage that you want to make money with a service, but please clearly indicate that right from the start and don't trick your potential customer into something. This is in my opinion fraudulent behavior, which I cannot endorse.