If you use the word "bienvenue" only you have to write it with an "e" at the end. It is a short way of saying "Je te/vous souhaite la bienvenue" or "Bienvenue à vous" so there is always 'e' when the word is used alone.
Ex: "Bienvenue"
If you use it in a sentence, you should write it as follow, depending on who you talk to:
"Soyez le bienvenu" (when you refer to a man)
"Soyez la bienvenue" (when you refer to a woman)
"Soyez les bienvenus" (when you refer to a group of men or a group of men+women
"Soyez les bienvenues" (when you refer to a group of women only)