No way to tell what the "terms" are for Beta testers. Usually a Beta test tries to give the testers use of the program in "as close to real life" conditions as possible, so they might expect you to order and pay for downloads just like "real people" would.
They would, probably, give you a chance to give them periodic "evaluations" of how well things work; and would be likely to ask you to make some kind of comments (answer a questionnaire?) at the conclusion of the test.
If you think you might be interested, I'd suggest an email to Mr Sonneborn to ask him how the beta test will work, and what you have to do to volunteer. He probably has at least a form letter he can email back to you.
Smithsonian is reputable enough you shouldn't have to worry about getting a lot of spam if you contact them, although actually participating might get you a newsletter offer or annual "begs" for contributions for a while. I get a very minor bit of that from my magazine subscription, but they're not pushy.
I probably won't inquire, since the amount of downloading I do is so minimal that I couldn't give them much of an evaluation; although I'd like to see the system work for the rare times when I do want something.