Simpler; not necessarily one share but more generally a holder of share(s).

The English term share is expressed in two different ways in French, 'part' and 'action', depending on the form of incorporation. A corporation (Société Anonyme) with shares that may be bought by the public, has 'actions', a partnership or Limited Liability Company (SARL) which is what this company is, has 'parts' rather than 'actions'. In order to deal with this implicit ambiguity in the English term share, I think often the term (share) is avoided in English in this context, because it seems to imply a tradable share, ie a share that may be sold to any member of the public.

The 'shares' in partnerships and LLCs cannot be sold to just any one, they must be another partner in the company or a person authorized by the other owners, and often implies an active rather than passive participation in the company. Please see the definition of Société à Responsabilité Limitée in the Wikipedia link below.

In this particular context, I think it might be best to avoid "owner of a share of the company" and go with the "holder of a proprietary interest, which I think is the most common way to express this in natural English (US) that doesn't sound like 'translated French".

Part can definitely be translated properly as 'interest' as well as 'share'.

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Note added at 9 hrs (2010-11-03 19:46:08 GMT)
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Pleas also see the article treat 'part sociale' in Wikipedia at the following link.

"fr.wikipedia.org/wiki/Part_sociale

and I really should have put a confidence level of "4" for this.

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Note added at 9 hrs (2010-11-03 19:58:15 GMT)
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Also, generally we say 'holder of an interest' formally in this context, but in casual business speech, we usually say "is a partner in...", the English word "partner" or course being derived form the French word 'part'.

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Note added at 18 hrs (2010-11-04 05:31:22 GMT)
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also, the definition of "proprietary interest":

Advantage, profit, right, or share held by the owner of a tangible or intangible asset or property with all associated rights.

Read more: http://www.businessdictionary.com/definition/proprietary-int...