Diplomatic "no"

I need to save this one for future reference, authored by my colleague, because it is VERY well-said!! (And I'm certain I can lift some of it for a similar situation in the future!)

(Name), our programs are offered to (intitution name) employees, which is why we ask for a (institution name) email to register. We maintain a limited enrollment in these workshops because they do involve hands-on activities (I note that this represents a physical limitation of our lab space as well).

As you mentioned, the (specific) workshop scheduled for next week is already full, so I guess that's an indication that there are indeed scarce resources involved. Frankly, I prefer not to have a member of the (institution name) faculty or staff displaced by an occasional hire. If she winds up actually getting a (institutional) email and everything that goes with it, I would say that's a signal of the institution's commitment that would warrant her participation, and at that point we would certainly welcome her attendance.

best,