subject says it all…
Probably a very hard question to answer. Sole proprietorships are almost always employee-owned companies, in that the owners usually work for their business, and a lot of partnerships are also fully owned by working partners. If you accept those as ‘employee owned companies’, then there are probably some examples dating back to the very first colonies on what is now the US east coast - many of them little more than a general store or other small retail venture. (Inns, taverns of some sort? You could describe any of them as companies I think.) It would be very hard to find much evidence of them in historical accounts, or to sort out which one was ‘first’.
If you’re talking about a more formalized co-operative corporation, then I’m not sure how far back you’d have to go.
Hope you don’t mind my WAG-ery.
Given that there have always been lots of one-person enterprises (or two-person enterprises if it was a married couple), there’s probably no simple answer for this. A larger enterprise which is the first example that comes to my mind is the nineteenth century Oneida Community:
They were a commune where all the members shared in the profits of the community. They manufactured a number of things. The most famous objects manufactured (although it was late in their history) was silverware. After the commune broke up, the enterprise was reorganized as a joint-stock company, and that survives today as Oneida Limited.
Incidentally, the 1840’s were apparently as big a period in American history for free-love shared-work communities as the 1960’s.