**Edit: I changed the method because the original one made the the body rotation very slow.
So I found this really neat trick that I found out.
I’m currently animating a rig character. (Note that I am using transform layers for this.)
For example: I have a rotation layer for the body and the head. But I want to it to move the arms as well, but they’re in a group layer outside the layer I have.
For this to work, you to make sure you put the arms in a group. Then select the rotation in the sets menu, and in the properties right click on “amount” and convert it into an integer as seen below:
An arrow should appear next the value you’ve converted.
Next, select the group you put the arms in. Open the group by clicking the drop-down arrow and add a rotation layer (right click, New Layer, Transform, Rotation) and do the same process.
Click on the arrow next to the value, and you’ll see a link. Right click and export the value as shown in this image below:
Name it whatever you like and click "export."
In the library you should see the value by clicking the arrow on ValueBase Nodes.
Now go back and select the rotation layer for the body. Click on the arrow next to the amount value you’ve converted and you’ll see a link. Select it and the link in the library (make sure they both have the value of 0), right click on the unexported link and click "connect."
With this done, you should be able to now rotate the body with the arms.
Hope this helps and have a great day!
***Note: Make sure you have the origin point of the rotation layer for the arms in the same spot as the one for the body so they don’t move out of place when animating.