Starfleet’s Prime Directive forbids interference with the development of intelligent species more technologically/scientifically primitive than the Federation member races, and (I presume) within Federation space or outside it. This rules out any conquest, colonization, or exploitation of them. It also rules out any form of officially sanctioned missionary work, whether religious or political, i.e., teaching them about democracy.
But it would also seem to rule out any form commerce with them. If they deal with Federation traders they will at least be made aware of the existence of Federation technology, which cannot help but affect their development.
But, as we’ve seen more than once on the shows, the Federation is a civilization where private merchants travel around at warp speeds in their own private spacecraft. Does it apply to them, and if so how is it enforced? And what about private, not-officially-sanctioned missionary work?