Formal:
* Bore da i bawb. (This is the most common and formal greeting.)
Informal:
* Bore da bawb! (This is a more casual greeting, suitable for friends or colleagues.)
More informal, playful:
* Diwrnod da pawb! (This translates to "Good day everyone," and can be used in a more informal setting.)
It's generally considered more polite to use the formal greeting "Bore da i bawb" unless you are very familiar with the people you are addressing.