Formal:
* Nimefurahi kukutana nawe. (This is the most common and polite way to say it.)
* Kumekuwaje? (This translates to "How are you?", and is a polite way to start a conversation.)
Informal:
* Samahani, nimefurahi kukutana nawe. (This is a less formal version, including "excuse me".)
* Hongera kukutana nawe. (This literally translates to "Congratulations on meeting you", and is used with friends or people you know well.)
To add emphasis:
* Nimefurahi sana kukutana nawe. (This means "I am very happy to meet you.")
To respond:
* Mimi pia. (This means "Me too".)
It's always best to err on the side of politeness and use the formal greeting unless you're sure the situation is informal.