Formal:
* Buongiorno a tutti e benvenuti alla mia presentazione. (Good morning everyone and welcome to my presentation.)
* Benvenuti a tutti. Sono lieto di presentarvi... (Welcome everyone. I am pleased to present to you...)
Informal:
* Ciao a tutti e benvenuti! (Hi everyone and welcome!)
* Bentornati! (Welcome back!)
Note:
* "Buongiorno" is used for "good morning" and is appropriate until noon. After noon, you can use "buongiorno" or "buonasera" (good evening).
* "Sono lieto" (I am pleased) is a more formal way of saying "I am glad" or "I am happy".
* "Bentornati" can be used if the audience has already attended a previous presentation.
Choose the greeting that best suits the context and your audience.