Formal options:
* Mudita: This is a Pali word meaning "joy in the well-being of others." It's often used in Buddhist contexts, but has gained popularity in secular settings as well.
* Freudenfreude: A direct translation of the German "Freude an der Freude" (joy in joy), this word is less common than Mudita but carries a similar meaning.
* Benevolent pleasure: This more descriptive phrase emphasizes the positive feeling derived from others' good fortune.
Informal options:
* Happy for them: This simple phrase conveys the sentiment directly.
* Sharing in their joy: This emphasizes the sense of connection and participation in their happiness.
* Feeling good for them: A more informal way of saying "I'm happy for them."
Ultimately, the best word or phrase to use depends on the context and the specific nuance you want to convey.