There is a blue button labeled "Share" in the upper right-hand corner. clicking that will allow you to share your document with others, and assign permissions (Edit, Comment Only, or View Only) to the people that you share with.

Sharring a document with another person gives that person editing permissions by default. If you want to share your document, but not allow people to edit it, you must manually set permissions to either Comment Only or View Only for each person that you add.