Share With Others
Open your document and click on Share > Share with others
You will see this screen below. Enter the email address of the person you wish to collaborate with. When you start typing, it will also show you a list of all Members and Guests in your team.
Afterwards, click share. The recipient will receive an email to edit the document.
How do I know who or if they are editing the document with me?
You will see their avatar appear at the top right next to share:
Click on your avatar to bring up settings. Hover over their avatar to see their name.
How do I see all people who have collaborated on this document?
Click on Share > Share with Others and you will see a list of people the document is shared with.
What permissions do Guests have?
Please see this article here on permissions.
Keep in mind that if you Undo while there are multiple people working on the same document then this will just work for your individual edit changes, and you can Undo only until you hit someone else's edit. Otherwise - undoing another users' work could lead to major confusion. Imagine that you are writing some text and your colleague tries to undo his own image edit, which consequently removes parts of your text!
Have a look at these articles for related info...