The basic political unit of a feudal world is the oath of fealty that binds lieges and vassals together. Out of these personal relations grow the larger political units, such as realms.
Anyone can swear fealty to any position, settlement, or place, at any time. These oaths will require the acceptance of whoever holds that seat, so if the seat is empty an oath can never be accepted.
A lord of an estate controls the realm membership of their estate and can change it at any time, but only to other realms that they are a member of (see below). Places of interest will work in the same regard in a future update.
First Ones are considered to be members of a realm if one of the following is true:
The game does not enforce a strict hiearchy outside of leige-vassal and realm-subrealm. What these oaths mean in a realm depend on the hierarchy of the realm as defined by the individuals within it.
That means you can join a realm by swearing an oath of fealty to a seat within the realm, provided there is someone holding the seat to accept it. The liege-vassal relationship binds knights who hold no estates of their own to realms, and they will join and leave realms with their liege.
It also means First Ones can become members of realms by inheriting or being granted estates within a realm. When granting an estate, the First One doing the granting decides if realm membership is passed along or not.
You can not join a realm by conquering an estate.