Why is it easier to carry a person while spinning than not spinning?

Normally, lifting someone up while stationary requires significant activation of the anterior deltoid (a relatively small muscle). When spinning, that force translates to back muscles, which are significantly larger/stronger, and your deltoids have to do less work.

An analogous situation is taking a weight and suspending it from a rope. Holding it stationary in front of you requires your bicep and anterior deltoid to activate, but if you start spinning yourself around, the centripetal force (+tension) keeps the weight elevated and now you just need to provide the centrifugal force inward to prevent it from flying away from you, which means pulling in a more horizontal direction, and so your bicep can work with your back rather than your shoulder.

You still need to provide an upwards force equal to their weight regardless of your motion. If you are spinning then your arms also need to provide a centripetal force component. So really spinning requires a larger force supplied by you.

The only explanation I can think of that makes it seem like you are providing a smaller force is that when you spin the person you are using different/more muscles than when you are just holding them, thus giving the appearance of a smaller force due to less effort of given muscle group. This is explored further in this answer. However, less effort is not equivalent to smaller force overall.

So the case is when you hold her you lean somewhat back and so that frictional force helps you. So you pull her a little towards yourself and that takes an extra force.

But as you start rotating then you have to also provide an extra force to provide her a centripetal accleration so it should take greater force. enter image description here

But as you have said that you are finding it easy then you might be spinning her and because of spinning she has a tendency to fall away but because of centrifugal force your arms get a better grip and the frictional force becomes more effective So there might be a role of friction.

UPDATE- but according to the newton's third law "every action has a equal and opposite reaction". The force would act on us even but mightinvolve different muscles. For a rough idea the skinof our hands are getting the reaction force, we just have to stiff our arms(joints with the body), This will provide our chest a force that could be balanced by tension force we develop in our body(rough idea given in this answer)

So role of friction is making different muscles to do the same task and hence you feel that she is lighter.

Image Source- Google