Everyone knows Google Apps, but many of us are using them in a one-off fashion. However, used as a suite, Google provides via Gmail, Hangouts, Calendars, Docs, Sheets, Slides and more many of the features project management and collaboration software offer. Like some other apps it requires a connection to the Internet, but it's accessible on most devices.
Users can work on the same document at the same time and see changes in real-time. They can also create hangouts for group chats or video conferencing.
Google offers Apps for Work with more storage, business email addresses, video and voice calls for $5 per user per month. For $10 per user per month, you get unlimited storage and additional administrative tools.