I don't have the da 21 (although it is on the wish list), but I think it would be great for group shots. A lot of people love that lens. I do have the Tamron 17-50, and it is wonderful for indoors and groups. I especially like that I can go to 17mm when things get really tight. They can both be had on the used market for a good price.
But, if you get the Tamron used, make sure it's a reputable dealer or someone here. Actually, that goes for any used lens, but I mention the Tamron because I've read that people have received bad copies. . I had a bad experience buying from an eBay seller with no selling history to back him up. I ended up sending it for repair because I had an event coming up and didn't have time for an eBay dispute. But since I've gotten the repairs, it has been a work horse, and I'm so glad I have it!
If I had a choice between 24mm and 21mm for that purpose, I would probably go with 21. Some may disagree with me though.
Just my 2 cents, hope it helps.