If there's a group of 3 or so of you in a tavern I can foresee Samm overhearing you chatting about travelling somewhere, and offering his services in protection, as long as he can get a cut of what is found, or some other payment. Even if its just around the Undercity, its a dodgy place, and considering you're all new you might need a hand against some muggers etc.
As for what group it is in the pub, well perhaps Travok and Willow met on the ship, and then met Thorn in the pub - since Thorn's quite charismatic he might have struck up a conversation with them? Perhaps Kadal knew Travok, and it was Kadal, Travok and Willow who are in the pub? Lots of options there really.
As far as knowing a Dwarven Shaman - whilst he may have met Travok before on one of his travels, he's unlikely to remember him, unless there was some major storm Travok calmed or similar. I'm not sure how viable it is for Travok to have done something like that considering he's still discovering the spirits and his powers, so I don't think Samm would know him enough to be adventuring with. However, if Travok is part of the group of people chatting in the pub who Samm overhears and offers his service to he might
recognise Samm and mention that he's seen him work before, and that Samm seemed reliable. Mind you, I don't know if a sailor would have made any more impression on Travok than Tavok would have made on Samm, although Travok may have a better memory.
My original reason for adventuring, not yet written up, was basically going to be - his latest job had a big pay-off but he came too close to getting caught, and no doubt executed, by the Rahyr for his liking (not as an individual, just as another guy breaking the law). However whilst on it, he met another person who had done a bit of adventuring, and suggested that Samm try his hand at a bit of freelance work. Bigger cuts, less run-ins with the law (usually), and more choice over what he does day to day, rather than signing a contract and suddenly finding himself in the middle of a war he wasn't expecting.
The Undercity would probably be Samm's 'home' as much as any place could be i.e. it would be the city he felt most at home in, and was probably born in - the port he aimed for in between jobs. So the reason he's in the undercity is because his current job just paid out, and now he's drinking, gambling and weighing up the idea of 'freelance' mercenary work. (Spoiler - he's going to decide to give it a go
EDIT:// Promise to stop editing now.
EDIT:// Character/player confusion, which is now fixed, making the whole post less confusing hopefully.