They may be paid by the various campaigns too, but probably not for the express purpose of getting on focus groups. I would expect the one who have better skills at oral persuasion are encouraged to apply for focus group slots too.
At any rate, the groups are presented as a "random" or "normal" cross section, but they are not. They are self-selected in the first place, then filtered by the press. Filtering does not include excluding people who do paid or volunteer work for a candidate.
Makes sense. Thanks!