The law defines an apartment owner as the person entitled to an apartment right (article 5:106(5) BW). Every apartment owner is a member of the VvE by operation of law; if you sell your apartment, your membership also ends by operation of law (article 5:125(2) BW).
As an owner you may use your private part yourself or let someone else use it, for example by renting it out (article 5:120(1) BW). You may make changes in your private part without the others' permission, as long as they do not harm another part, and you report them to the VvE straight away (article 5:119 BW). The regulations may set different rules.
In return, you contribute to the shared costs and debts in the ratio set by the regulations (article 5:113(2) BW). Anyone who buys an apartment right must notify the VvE in writing without delay (article 5:122(2) BW).
See also: Owners' association (VvE), Apartment right (appartementsrecht), Private section (privé-gedeelte), Occupant (gebruiker), VvE contribution (service charge)
Legal basis: Boek 5 BW, art. 5:106, 5:113, 5:119, 5:120, 5:122, 5:125