I drive coworkers to the fields and they chip in for gas. Is that covered?
Generally yes, as carpooling. Sharing actual costs between colleagues is not the same as carrying passengers for hire. Where it becomes a set payment per person per day it starts to look like livery, which personal policies exclude. Tell us the arrangement and we will confirm which side it is on.
What limits should I carry if I take several people?
Considerably more than the state minimum. One accident with four passengers can produce four injury claims against a single limit, and the minimum was never designed for that. Raising liability is the single most important change for a household that drives others.
My employer asked me to bring people. Does their insurance cover it?
It may, and it may not, and the answer depends on the arrangement. That uncertainty is itself a reason to settle it in advance rather than after a crash, and to make sure your own limits are adequate either way.