Slack is a popular messaging app similar to IRC (Internet Relay Chat) that's oriented toward small-team collaboration. It's free to use and available for Windows, MacOS, and mobile. Subscribe to Slack Plus for features like guest access, unlimited message archiving, guaranteed uptime, and increased cloud storage (20GB, versus 5GB in the free version).
Slack doesn't skimp on the free version: You can get Slack for Windows, Mac, mobile, and within a Web browser. And in any of those versions, you can make two-way voice and video calls, apply optional two-factor authentication, store up to 10,000 messages, and use up to 5GB of cloud storage per user without forking over any money to Slack. This helping of features could easily support a small business.
Saves messages when you're offline: Slack's cloud keeps copies of all messages its users send, so when you log in, you can quickly catch up on any discussion that you might have missed.
Integrated support bot: Slackbot is a conversational AI like Siri that can answer basic questions about how to use the service, as well as provide links to Slack's website for detailed answers and tutorial videos. This approach is more efficient and informative than paging through a manual or a FAQ.
Occasional start-up quirks: Every once in a while, Slack may present you with a nearly empty window when you open it, save for the menus at the top. If you click the Window menu, select "Sign into another team," and give it the email address associated with your Slack account, the service will send you an email containing a link that will let you log in to the discussion groups that this account has access to. Clicking the link should reset the Slack interface and let you resume using it as normal, but this process can be a hassle, especially when time is of the essence. Thankfully, it doesn't happen often.
Slack can scale smoothly from a handful of users to a sizeable business, and the free version will give you most or all of the features you want. However, its main competitor, HipChat (Windows, Mac, iOS, Android), is evolving just as quickly and is worth taking a look at, too.