Site mod here: I'm not very settled in my answer to this, but will say yes for now. Please provide many details so people can assess the role easily.
Advertisements on Lesswrong (like lsusr's now-deleted "Want To Hire Me?" post) are good, because they let the users of this site conduct mutually-beneficial trade.
I disagree with Ben Pace in the sibling comment; advertisements should be top-level posts, because any other kind of post won't get many eyeballs on it. If users don't find the advertised proposition useful, if the post is deceptive or annoying, then they should simply downvote the ad.
We have had job postings in the past. I think job postings that are on core topics like AI Interpretability research are good, but job postings for random programmer jobs (especially by new users) wouldn't.