No, it’s an undocumented subset of Xtend with then some, but not all of the objects from the smarthome framework exposed. Running the v0.8.0 version of Designer is about the best you can get, but even it is far from reliable. The JavaDoc from the EHS project are for Java which is definitely not what the rules interpret.
I ran into the same question and it seems indeed, that memberOf(Group) is not support in Xtend. My log says: 'memberOf' is not a member of 'org.eclipse.smarthome.core.library.items.NumberItem'
However: It would be great to have a way to filter member of e.g. subgroups of a larger group of items.