You can create a new link group for any document that is not already a member of a link group. Creating a link group can be done in the Links tab of the Metadata dialog box in the Folder Browser, or in the Links tab of the Metadata Pane in the Document Viewer.
If a document is already a member of a link group, you will not be able to create a new link group for that document, but you can remove the document from its current link group and then create a new link group.
When a new link group is created from a document, the document will be added as the first member of that link group. If you want to add the document to another, already existing link group instead, you will need to do so from within a member of that other link group; see Adding Documents to a Link Group for more information.
To create a new link group