I've only got 2 invites, but both of them gave me a list of dates to choose from. You don't have to ask about available dates or times unless the list they give you doesn't have one that you can attend. You could ask about accommodations, or you could just look through their website to find a list. AZCOM, for example, has a published list of hotels they suggest you stay at.
Also, I wouldn't bother thanking them for the invite. I did a search about this a couple of weeks ago and I found
http://forums.studentdoctor.net/showpost.php?p=10275657&postcount=3, which I 100% agree with.