An Owner may share documents with existing users by granting them access as Editors.  

The possible levels of Editor access that can be granted are:

View Only

Users can view the document.

View and Edit Users can view the document and make changes on behalf of the Owner/Approver/Submitter.
View, Edit and Submit Users can view the document and make changes on behalf of the Owner/Approver and then forward it to a next stage in the approval process.

The allowable levels vary depending on the document types.  For research grant proposals, all three levels are available.