I don't think they would simply be a mercenary, we already have one in Iron Bull. I'm willing to believe the human is either the younger child/sibling or servant of a nobleman who attended the conference for the prestige such a thing offered.
The surface dwarf might have gone as a smith: People have to travel a long way to this conference (from Orlais to Ferelden is a few weeks, I understand), so someone would be there to repair any broken weapons or some such.
The Dalish might have been offered to attend simply because the Divine hoped the peace conference could help simmer tensions in Halalmshiral (She strikes me as the type who would go for the masterstroke)
I don't know about the qunari, though. Couldn't imagine what use a Vashoth would be.