From page 7 of the
Warhammer Age of Sigmar Core Rules Errata:
Q: A Sylvaneth Gnarlroot Wargrove can include an Order Wizard, and a Sylvaneth Winterleaf Wargrove can include an Order unit. Are such units allies?
A: Yes they are. However, because they are part of a Sylvaneth warscroll battalion, they do not count towards the number of allied units you can include in a Sylvaneth army, and their points will not count against the number of points spent on allied units for a Sylvaneth army in a Pitched Battle. Note that although they don’t count against these limits, for all other rules purposes they are treated as being allied units, and therefore can’t be given artefacts of power, know spells from its spell lores, and so on.