Go to the Contact, record click on the Document sub-tab, and hit Select Document to pick the document to associate with that contact.
OR
From the Document record:
Go to the Document in question, click on the Contact sub-tab and link all the people you need to.
To link an Activity (e.g. a Meeting) to more than one Contact;
Go to the activity itself, click on the Contacts sub-tab, and link all the people you need to.
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