Les / os doy / damos la más cordial bienvenida.
Les doy ... / os doy (if it's one person speaking).
Les damos... / os damos (if one person speaks, but as part of a group who says welcome to the other people)
Sé bienvenido, a al grupo.
Sean bienvenidos, as al grupo.
Sed bienvenidos, as al grupo.