This depends on gender and number.

Bienvenido- speaking to one male

Bienvenida- speaking to one female

Bienvenidos- speaking to a group of males (two or more) or a group of females and males (mixed)

Bienvenidas- speaking to a group of females (two or more)

Examples:
Mia says to Jake, "Bienvenido a Puerto Rico."

Elijah said to Elena, "Bienvenida a Cuba Elena."

Joseph welcomed his friends Jake, Tom, John and Paul.        "Bienvenidos amigos a Bolivia."

Danielle welcomed her two friends, Mia and Alya. "Bienvenidas amigas"