Tony M May 13, 2012:

Right! Now with proper context it makes sense!

So 'partner' is in fact qualifying 'extranet' — I'd say 'on' in that case: you are registering 'with' us, but you are doing it in a specific place, i.e. 'on' the extranet.

veratek (asker) May 13, 2012:

This is an extranet (part of a website) exclusively for their partners (of this company)

The company is inviting their business partners to register to keep abreast of what they are doing, news, etc. By registering, their partners can have access to the extranet area.

in French: ... en vous inscrivant sur notre extranet partenaire

Jenni Lukac (X) May 13, 2012:

I agree with Tony. It depends on the context.

Tony M May 13, 2012:

Context? The answer is that either could be correct — it all entirely depends on the context!

Normally, one registers with a body of some kind; but in the absence of context, this could well be referring to registering with [someone] by means of (= on) the extranet — the word order 'partner extranet' is a little odd (again, without more context to clarify); is it the partner's extranet? Or is the person actually registering with an extranet (which happens to be a partner)?