Maybe you could say something like, "We're planning to stay at the retreat center on Friday, Saturday and Sunday nights, and hope that our family and friends will join us. We'd like to treat you all on Friday night, and rooms for Saturday and Sunday will cost $30 per night."

I'd also include some more information about food and activities. Is food provided at the center, or will people have to plan extra money for restaurants? What is there to do?

I'd also think a bit about how you're going to collect money. I assume that you'll have to collect all the money, am I right? If so, I think I'd do it through a Paypal personal account, and send out invoices. Make sure you set a date a week or two before the wedding for final payments.

This sounds like a lot of fun, if you have a nice center and family and friends who are interested in spending a whole weekend together. Good luck!

Joann is right, don't put any information about accommodations on the invitation itself. Use a separate sheet about accommodations, or even a letter with all the information they will need (like directions and other events and activities of the weekend, not registry info). Joann's wording is good, or you could write, "We will be staying at the XYZ retreat center. It will be our pleasure to treat you for Friday night." Then just include the contact information for the retreat center and let them make their own arrangements if they want to stay the whole weekend (I'm guessing some will want to stay Saturday but not Sunday).

I don't think you need to include the information that the rest of you will be staying through Sunday night. This is a small group (I'm guessing that by "40 invitations" you meant 40 invited guests, not 40 mailed invitations); most of them will probably know anyway, and you can write a little note on the paper informing those who might not know. But if you are doing it in the form of a lengthier letter, you could write something like, "We will be staying at the retreat center through Sunday night, and we would be delighted to have you join us. Rooms are $30 per night, and we would like you to be our guests for Friday night." Then put the contact information for the retreat center.

I'm assuming that the retreat center will (a) allow people to stay for just one night, and (b) they will take reservations and payment directly from your guests, so you don't have to collect the money. Again, for so few guests (some will be in couples or family groups anyway), even if it's not something they usually do, maybe they will.

I definitely wouldn't write "donations of $30 a night would be appreciated." That would just confuse me. Donations to whom, you or the retreat center? Is $30 the cost of the room? For one night or two? Is it optional on my part to pay more, less, or not at all?

It does sound like fun! Congratulations.