Slack is a great tool for both large and small team. The simple interface lets you communicate and organize work with your colleagues, text or call an individual or a group, share, edit documents and collaborate with teammates for a project. It also allows integration of third-party apps like Google Drive, Dropbox, etc.
Zoom is another good tool to organise conference meetings online. You can run video meetings or even group or one-to-one voice calls when video calls aren't necessary. You can host webinars and share files during collaboration with co-workers.