If all the workspaces are in the same installation, select the workspace with the documents you want to share/move, click on the Documents tab, select a document you want to move/share, click on Properties, click on Workspace, check/uncheck the appropriate workspaces.
Unfortunately, I don't think you can do this for more than one document at a time.
(Like classifying documents, this should be an operation you could apply to many documents at once.)
If your source and target workspaces are in different OG installations, I suspect you will have to download from one workspace and upload into the other.