I had an extra layer of a cake a few weeks ago so I froze it. Now I was going to bring it over to a friends as a coffee break treat. Do I let it fully thaw before decorating? Or can I decorate it frozen, this sounds easier to me really, less crumbs

I decorate frozen cakes all the time, just watch you frosting, sometimes it will pull away from the cake. I have had it bubble a couple times too but you can smooth them out. It is easier to decorate, with less crumbs.
Some say never to freeze cake but I do.

I have decorated frozen cakes, but when they thaw they tend to create moisture beads on the icing which can make it not crust well. On the other hand, freezing it helps lock in the moisture in the cake so it is super moist.

you might also have a problem with the frosting cracking as the cake thaws. To avoid this, I have always stabbed the cake a few times with a knife before I frost it. Or you could just avoid the whole issue and thaw the cake before you decorate it. Should only take about 30 minutes on the table. I will only frost a frozen cake if it's absolutely necessary.

Thanks guys, cake is out and thawing, I'll decorate after I get my son from school.