Say where they are, if you can
Give the position or a range. If you cannot, say how the list works.
Be honest about timing
Give a rough window and say it can change. Do not promise a date you do not control.
Explain how offers are made
Tell them how you will contact them and how long they have to say yes.
Offer an alternative
Suggest another date, location or product that is open now.
Ask them to keep details current
A wrong email loses a spot. Ask them to confirm how to reach them.
Close the loop later
When a spot opens or the list moves, tell them, even if the news is no.