Bienvenido vs. bienvenida
The Spanish words bienvenido and bienvenida both mean welcome as a greeting. The difference between them is based on grammatical gender, matching the person or people being welcomed.
Bienvenido
A1This is the masculine form of welcome. It is used when welcoming a single male, a group of males, or a mixed-gender group. In Spanish, the masculine plural is the default for a group containing both males and females.
Carlos, amigo mío, bienvenido a la ciudad.
(Carlos, my friend, welcome to the city.)
Señores, bienvenidos a la reunión.
(Gentlemen, welcome to the meeting.)
Hola a todos, bienvenidos a nuestra casa.
(Hello everyone, welcome to our house.)
El hotel tiene un letrero que dice: Bienvenido.
(The hotel has a sign that says: Welcome.)
Bienvenida
A1This is the feminine form of welcome. It is used when welcoming a single female or a group composed entirely of females. It is also used as a noun meaning a welcome party or reception.
Isabel, qué bueno verte. Bienvenida.
(Isabel, it's so good to see you. Welcome.)
Chicas, bienvenidas al club de lectura.
(Girls, welcome to the book club.)
Señora Presidenta, sea usted bienvenida.
(Madam President, you are welcome.)
Organizamos una fiesta de bienvenida para la nueva directora.
(We organized a welcome party for the new director.)
Summary
In short, the choice depends entirely on the gender of who you are welcoming. Use bienvenido for a man or a mixed-gender group. Use bienvenida for a woman or an all-female group. The ending of the word must agree in gender and number with the person or people it refers to.