Hi Steve - Welcome to the forum
Six of us shared a Tipi last year, we had more space than i expected (six camp beds with plenty of room for Rucksacks and still 1.5m square left in middle

) I'll try to answer your questions:
1. No, they are not assigned in advance (at least not last year), we had a few choices last year when we arrived at midday Thursday.
2, 3 & 4. When you get to "Tipi reception" you will give the name of the person who made the booking and they'll ask how many of you and you give the names of the 5 other people staying in each Tipi. So if you arrive Wednesday I shouldn't think you'll have a problem getting Tipi's next to each other. Two of us arrived later and when we got to the "Tipi reception" we just gave the name of the person who had made the booking and they checked that our names had been put on the list.
In your question 3 you ask about friends getting in to the Tipi Village - if you mean friends who are not staying in the village (ie camping on the main site), they can'y get in - only wristbands that you get given exclusively when you stay in the Tipi Village. They did check when you walked in and when we were queing for the showers they did a "run down the line" ask sent people away who had snuck in.
You'll have a great time
Roll on June!