This is what I believe but it is an unofficial answer based on my own experience.
A "trusted user" is a TB forum-only distinction given by the forum moderators based on your posting history, forum conduct, and forum contributions in these forums. I think loosely you have to have around 200 posts. But that isn't the only criteria. There is a subjective element to it.
I never "tried" to be a "trusted user". I was actually surprised by it. I guess someone felt my ongoing contributions here on the TB forums were appreciated enough and it was granted to my account. I took it as a big compliment.
I have no idea how others got their "trusted user" title but I imagine they were selected and appointed by someone with TB.