Spots get filled in the order people join.
If player #1 joins, and #2 directly after that, they will be next to each other anyways. Try it for yourself with a friend.
(Disclaimer: I'm assuming nobody left. The whole system in solo ew is a starting island, then all the island to left (or right, I actualy don't know) will get filled up one by one. This also means that if one person leaves, the next person joining will fill up that spot, not putting him into the 'circle'. The next person will continue the circle.)