Yep, as folk have alluded to, in Early Access there's a workaround for a full custom party where you basically pretend to set up a multiplayer game and then take control of all the characters.
But though we do expect there to a (more official) solution or solutions for custom parties in the full release we don't yet know what they will be. There was a tantalising hint in a tweet showing Withers, who joined our camp in early access to provide resurrection services, also offering to bring hirelings from beyond the grave to help us (possibly in return for soul coins, some have theorised) and while we don't know for sure that those hirelings will be customisable characters that seems a reasonable bet.
Whether that will be the only option, if it is one, or if the existing workaround to have four custom party members from the start will work in the full release, is to be confirmed.