1. Go to the document you want to share.
2. Click the blue Share icon (upper right corner).

3. In this window you can modify the general sharing settings of the document by clicking Change... next to the first item on the list under Who has access. There are five different options:

 Note: you can only change the general sharing settings of the document if you have editing rights to it!

4. Under the general settings are listed all the users that have access to the document. 

Type in the box Invite people  the users/groups you want to share the document to. You can choose the access level for each user/group after you have added them on the list. You can also choose whether you want to let them know about the share via e-mail.

5. Click Done and the sharing is done.