Can someone give a first-timer a quick rundown on what's involved in replacing a standard safety with an ambi-safety(DPMS)? Does the whole trigger group need to be removed?

No.  Just remove the pistol grip.  Watch for the selecter switch detent and spring.  The selector (saftey) switch should slide out the left side with a little twisting and manuvering.  Put the new one in and the right side screws on to what was the flush end of the old one.  

Re-install detent, spring and grip.

Halfcocked, I happened to have the same question as I had also purchased a DPMS ambi-safety.  Your instructions made the installation a snap. There are probably a number of individuals who have the same question that you more experienced members provide answers to in any given post.  I just wanted to take this opportunity to thank you and the other members for the information you provide us.