Access to workspaces and datasets

Hello, 

Is a Power BI  Pro license needed just to access a workspace? 

My users are able to access reports that I shared with them without entering.  However regardless of the access level I give to the users, when they try to access the workspace, they get the error message

I am trying to acheive the scenario where I can upload a dataset to Power BI, share it with other users and give them permission to create reports. They can then send me the file to publish to Power BI. How can this be achieved in Power BI:

1. without getting Pro licenses for each report developer and  

2. without supplying the users with credentials to the data sources?