I'm making an animation with a guitar and a G3-actor. I want the hands to be in front of the guitar, but how do I do that?

All you should need to do is take your character into Composer mode and "Attach" the guitar to your character. You can then use Layer Manager to make everything look right. I would advise taking a look at the Layer Manager tutorial below which will explain how this all works.

https://www.youtube.com/watch?v=Y0ym67saMQE

                                                                

Peter
Forum Administrator

www.reallusion.com

An by "attach" you mean "pick a bone and add as a sprite"? The icon "attach" is greyed out, and I can't drag/drop any image to composer mode... 

You have to create and save the prop first.
In composer mode of an actor you add the prop from library, click the attach buttom and choose a bone.
Then use layer manager in scene mode. The prop will follor the bone it's attached to, but can also be animated by itself.