Your description suggests the document has acquired some of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

That said, it's not clear what you mean by "only seems to happen when more than one person is added to the list of users". Are you perhaps referring to the exceptions you can apply to 'No changes (read only)' restrictions?

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]