I played a couple hours of co-op with GamesRadar's Austin Wood, and we were impressed by how smoothly summoning worked in Elden Ring despite the size of the world and how much freedom it gives you. There's a password system if you only want to play with friends, and using that we were able to see each other's summon signs just like in any previous Souls game. When we summoned in the open world, we didn't run into any restrictions on how far apart we could roam, though you're not allowed to ride your spectral steed while in co-op.
There was one disappointing limitation: when you find a dungeon to enter from the overworld, it's seamless in singleplayer, but throws up a fog wall in multiplayer preventing you from entering in co-op. Your friend has to leave, and then you can walk into the dungeon and resummon them once they walk or fast travel to it in their own world. I wish that barrier wasn't there, but I understand why it is: they treat the dungeons as separate summon areas, and with fast travel from anywhere on the map it's still really easy to pick up with just a short delay.
Multiplayer in this game is clearly going to be a major initiative: it actually has its very own dedicated entry on the main menu now.