G and D share the same note. Many people try to hold the strings down on the frets all of the time like you would on a typical guitar strum. It's far too much work. Before the pick strikes, quickly depress the strings and let up as soon as the pick strikes all of the strings. In other words you're quickly dampening or muting the chord once it is played.
Most people are afraid they'll make too much noise.
I'm here to tell you that with a guitar, banjo and bass there's not a chance in hell you'll be too loud. However, you have a role to contribute and you can't do it unless you're heard. Long OK, ya get what you pay for. Here are 10 good basic beginner tips to go home with. Buy an electronic tuner to keep yourself in tune but practice tuning your mandolin on your own.