[support] Collaborative editing only on some nodes?

Hello,

I have a Drupal 4.7.3 website where I must allow some users to edit some nodes together. This should not be the default, and it should be possible to do even on pre-existing nodes: that is, user A creates his ten personal nodes, maintains them only by himself for six months, then decides to give write access to node #3 also to user B and write access to node #8 also to user C.

How can this be done in Drupal? Which modules or settings should I use, and what is the relevant documentation? Remember I want this to be the exception, not the rule, as it would not be needed for most nodes.

TIA, D.