Can characters join units?
First page under Warscrolls &amp; Units heading.

"A unit can have one or more models, but cannot include models that use different warscrolls."

No.  No warscroll may ever join another war scroll.  Characters are always alone and may never join units.  The only exceptions would be if a specific character has an ability on their warscroll that allows them to join another warscroll.  Goblin fanatics (not a character) are the only example I can think of offhand, but they are actually hidden within a unit.