I've searched and searched, and can't decide on a 400 speed film in 35mm for portraits. I'm leaning toward either Neopan 400, Delta 400, and Tri-X. Still learning, so I currently use DD-X with Delta 100 and Fuji Acros.
I will be mostly shooting with indoor available light, occasionally using the built in flash on my Contax NX if needed.
Well, the best thing is to use something you're familiar with. I would suggest using HP5 Plus, but it wouldn't make sense for you to use that for the first time on important family reunion shots. Whatever you choose, shoot a few test rolls first so that you're comfortable and confident with it. The Neopan 400 does seem to be quite popular - I haven't tried it yet myself.
It's really a personal preference thing. But, if you like Delta 100, you'll probably like Delta 400, as well. Works well in DD-X, too.
I agree with Kevin, though - shoot and process a test roll if you can, before putting yourself in the position of everyone "depending" on you for pictures.
I'd lean more towards Tri-X, but then I'm particularly biased towards Tri-X for a lot of things. Hp5+ is good and so is Neopan 400 and the rest. If I want a film that I know will give me the goods though, it's Tri-X every time.
I would suggest using HP5 Plus, but it wouldn't make sense for you to use that for the first time on important family reunion shots. Whatever you choose, shoot a few test rolls first so that you're comfortable and confident with it.
I pulled up the PDF on HP5 Plus, and DD-X is the recommended developer for Best Overall Image Quality at 400-3200 and for finest grain. The trip is in January, so I'll order up some now and do a few trial runs and see what happens. To all who replied, you have my thanks and appreciation!