Calanrin was sent from Rivendell to aid in the quest to rescue the dwarves' missing companion and recover the information that he sought. She is a capable scout that cringes at every creak and clang of the dwarves' armor.
Eldarion has aided the dwarves in the past and has even dared an excursion into Moria. When he received the message that their companion was missing, he journey to join their party as soon as he could.
Hamfast is a rare hobbit indeed. He can grow a beard, which is an odd start, and he isn't content staying in one place, odder still. He met Eldarion some years back after getting into a bit of trouble and now the two journey together from time to time. It was Eldarion that brought him to Moria knowing his skills would benefit the group.
Salis, along with Vidar, took part in the planning of the original expedition in which their companion was lost. She feels responsible for the failure of that first attempt to delve into Mazarbul, so this time she is going in as well. This directly defies the order of Dáin Ironfoot which decrees that no dwarf shall enter Moria, but she's willing to risk it.
Ulfrun is the only member of the original expedition to return. She knows the path they took into Moria and desires vengeance on the orcs that waylaid her and her companions. In spite of the horrors that she encountered the first time, Ulfrun demands to return with the rescue party.
It burned at Vidar that he couldn't go on the first expedition after the planning and effort that Salis and he put into it. Having the expedition fail ending in the death or capture of the members of that party scorned him further. Now he is going to enter Moria, find his missing companion and discover the secrets that Mazarbul contains.