Hello. Just say this to a man:

- Bienvenu.

This to a woman:

- Bienvenue.

This to 2 or more persons, both men or mixed gender:

- Bienvenus.

This to 2 or more female persons:

- Bienvenues.


Yours,
Saïd.

hi, you can say
bienvenu (e) à vous (a person you respect exemple:guest, boss, ....etc)
bienvenu [(e)girl] à toi (one person as your freind, father, sister,...etc).
bienvenus (es) à vous "plural/many persons".

Soyez les bienvenus!

Welcome -> Bienvenue

It is ALWAYS "Bienvenue" and never "Bienvenu" or "Bienvenus" or "Bienvenues" !! Others are wrong ;)

Mewfree 's answer seems to contradict with Said's answer.

now.. i'm confused.