Getting a value from a node referenced in an entity reference field

I have two entity reference fields in an Article node, being 'Category' and 'Group'. Each category node already has its reference to a group. When a user creates an Article, sets a category and saves, I want the Group reference field to be automatically filled in by Rules, it should get the same value as the Group reference field of the selected Category.

I started off with this Rule:

Events:
After saving new content

Conditions:
Entity is of type Article
Entity has field, Parameter: Entity: [node], Field: field_group_category
Entity has field, Parameter: Entity: [node], Field: og_group_ref

And now I need Rules to get the value of the field_group_category, and look in that node to find the value of its og_group_ref field, and copy past that in the og_group_ref of the saved Article.

I have no idea how i do that in Rules...