I read about TESO quite alot, but i didn't find information about this. Let's say i'm Aldmeri Dominion, and we just had an epic battle, we conquered one of the greatest castles. Now i have to logout. After 10mins i'm back again but i'm in "another" Cyrodil and this castle is again in hands of my enemy.

I hope it will not look like this? Will we be connected to the Cyrodil instances randomly? What if i wanna play with my friend who is in another Cyrodil? I have to pay for gems like in GW2? When i play MMO, i'd like to know good guilds on my server, but i guess it will be impossible with Megaserver system.

You'll be assigned to a Campaign. Only way to change the Campaign your in is a High Alliance point cost. Otherwise you'll always be in the same Campaign.