Hi Eliza,
The way the default Opportunities Dashboard is set-up is to allow you to filter the report based on the available users in the Filter By User dropdown. You can see that here.
The easiest way to accomplish getting the report from all users would be to customize the screen and insert the same report again, just without this filter. That way you won't break the current functionality of the "Opportunities by Stage" by user. Just copy over the values from the original report, leaving out the AssignedTo filter. Make sure you input the OpportunityStageCategory filter, or the report will include closed opportunities.
-Audisho