I went looking for an answer to this question today, because I got a suspicious-looking request: a guest user with a name I do not recognize, requesting to be put on the Trusted List for two people famous enough to have Wikipedia profiles.
I've honored requests, and made them, and in both cases, the requests don't come out of the blue: we've encountered one another on Wikitree before. The profile manager can see how the requesting user and the profile subject are related. We've both worked on other profiles around the one in question.
An appropriate request will give enough information for you to know why this user is making this request, and will come from someone with a demonstrated interest that clearly includes this profile.
Giving the requester the benefit of the doubt, I asked him how we are related. (I will ask him in follow up, how he's related to the profile subjects, since they and I are not kin.) When I'm satisfied that he is who he says he is, and has the ability to meaningfully contribute to the profiles he's asked for access to, I will put him on the Trusted List. Putting up his own family tree would be a start.