Real-time collaboration is one of the strongest features of Google Docs, Sheets and Slides.

To collaborate with multiple users, click the 'share' icon and enter the email addresses of your collaborators. Make sure that you select 'Can Edit' for their permission level. 

As users make changes, you will see the changes appear with their name next to their cursor so that you know who is making the change. This real-time feature is only available when both parties are on an active Internet connection. For information about offline collaboration, please visit https://support.google.com/drive/answer/1628514.

To see a history of all activity for a given document, open the Google Drive app. Next to the file name that you want to review, click the 'info' icon ('i'). Browse to the 'Activity' section to see the edits with the most recent appearing at the top of the list.

https://support.google.com/docs/answer/2494891?hl=en