[eluser]TheFuzzy0ne[/eluser]
I don't think that MPTTree has been designed to work with multiple trees. Technically, each tree should have it's own table. I might be wrong here, but I'm sure that you can't manipulate multiple trees within the same table. A way to get around this, might be to give each user a top node in that tree (just under the root node), however, I'm not sure if this will cause problems if you're tree becomes too big.