Miembro vs. socio

The Spanish words 'miembro' and 'socio' both loosely translate into English as 'member'. However, they are used in different contexts and carry slightly different connotations.

Miembro

'Miembro' is a more general term for a member of any sort of group or organization, not necessarily implying any rights, duties or participation in benefits. It can refer to anyone who is part of a group, including members of a family ('members of a household'), body parts ('member'), or members of a team.
Soy miembro de la junta directiva.
(I am a member of the board of directors.)
Es miembro del equipo de fútbol.
(He is a member of the football team.)
Es miembro de nuestra familia.
(She is a member of our family.)
El brazo es un miembro del cuerpo.
(The arm is a part of the body.)

Socio

'Socio' tends to be used in more specific contexts, referring largely to members who have made some form of commitment (often financial) and are entitled to certain benefits in return. Common uses include members of commercial partnerships, clubs, societies or shareholders ('partner').
José es socio en una firma de abogados.
(Jose is a partner in a law firm.)
¿Eres socio de este club de golf?
(Are you a member of this golf club?)
Como socio, obtendrás descuentos especiales.
(As a member, you will get special discounts.)
Se convirtió en socio del restaurante, invirtiendo su dinero y tiempo.
(He became a partner in the restaurant, investing his money and time.)

Summary

'Miembro' and 'socio' both translate to 'member', used in different contexts. While 'miembro' refers to an individual as part of a group or team without necessitating any formal obligations or entitlements; conversely, 'socio' designates those members acknowledged as shareholders or partners having rights and duties often borne out of financial commitments.