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They are probably going to visit us a lot. What would they need to do in order to qualify for a multiple entry visa?
The 'Standard Visitor Visa' is multiple-entry by default so there is nothing special they need to do.
...how should they apply for the visa, since we do not have exact dates, and they can not book tickets yet?
Unlike other countries, the UK prefers that applicants do not purchase tickets or make bookings in advance of a decision. This is explicitly addressed in the guidance...
Should I give them some kind of letter or something for when they apply for the visa?
It's optional based upon what you wrote, but it's a good idea to include something that shows your status in the UK (like a scan of your T2 and a scan of your passport's portrait page). I am assuming that you are not financing their visit.