Formal:
* Bienvenue au spectacle! (This is the most common and formal way to say it.)
* Bienvenue à l'émission! (This is similar to the above, but more appropriate for TV shows or radio programs.)
Informal:
* Bienvenue dans le show! (This is more casual and can be used for a variety of shows or performances.)
* Bienvenue au spectacle ! (This is a more casual version of "Bienvenue au spectacle!")
You can also use a phrase like:
* Soyez les bienvenus au spectacle ! (This is a bit more emphatic and can be used for a grand entrance or to emphasize the welcome.)
The best way to say it will depend on the tone and formality of the event or performance.