Last weekend I took a couple friends to the local crag for their first time out.

Since I was the only one able to clean, others led and set the anchor, but on occasion if the leader were unable to reach the anchors, I would set the anchor and belayed the others up from the top using a grigri. That worked pretty well, though I'm aware that Petzl doesn't recommend using the grigri in such a manner (a redirected belay is preferred.)

I did notice that the DMM Pivot set up in guide/auto-blocking mode had a dedicated method for lowering — using a second biner to adjust the angle of the device. Are there concerns with doing so for lowering a second climber all the way to the ground?

Whenever lowering is mentioned, it's always in the context is lowering the second "a few feet" or so.

@devnull Short answer is "Yes".
Guide mode belay devices released by a 'biner are largely, due to the way the blocking works, an on/off proposition when it comes to the braking applied to the rope - it's very hard/all but impossible to control the lower well especially over long distances. In the "braking off" position, you're relying increasingly on strength for control, which gets worse the heavier the climber. There are various ways to mitigate this, but it required quite a bit of knowledge and practice to do safely.
This is not to say it's impossible to fully lower in guide mode, but not a great option.

@daanschone@pixelfed.social right! A prussik on the brake strand is definitely seems mandatory here.