This is a tricky one! The K1 provides work authorization for the first 90 days after entry. However, the only port of entry that actually provides a suitable stamp that satisfies HR departments is JFK -- and then you sometimes have to ask for it. Working for relatives or friends should not be a problem without the stamp (especially since you can apply for a social security number), but some employers will be reluctant to hire.

After the 90 days of your visa are up, so is your work authorization. As soon as you marry, you can apply for an EAD in your AOS package, but there is a delay of up to 3 months before the card actually comes through (there have been recent reports of this becoming quicker). Between the 90 days and receipt of your EAD, you're not actually authorized to work -- though USCIS don't seem interested in punishing infringements.

Afraid it's just one of the many illogical anomalies in the wonders of US immigration.