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bienvenue!.
in English we use the word "welcome" for all sorts of things (to welcome someone in, welcome!, you're welcome, etc), but in French it is a different word depending on the...
bienvenue !
or, soyez le/la/les bienvenu(e)(s) ! (agree with who is being addressed)
"To welcome" is accueillir (j'accueille, nous accueillons, etc.) and "a welcome" is un accueil.