It's possible that he's simply investigating how they managed to track him down to deliver the letter in the first place, which means they found him, which means someone managed to track him and he needed to find out how. 

Or he could have simply accepted the letter out of pure curiosity. 

As for what he'll do with the group, while he'll be fine with stopping Hollows from attacking people it is very possible he might try and pursuade the group to wait for Shinigami instead of finishing off the hollows - restraining instead of killing - so they can go and have their souls cleansed. He might appeal to their pathos. Failing that, he might even antagonize and work against them, sabotaging their hunts.