Formal:
* Comment vous êtes-vous rencontrés? (This is the most formal way to ask.)
Informal:
* Comment vous êtes-vous rencontrés? (This is still considered polite, but less formal than the above.)
* Comment vous êtes-vous connus? (This is a more informal way to ask.)
* Comment vous avez fait connaissance? (This is a more casual way to ask.)
You can also use these expressions:
* C'est comment que vous vous êtes rencontrés? (This is a very informal way to ask.)
* Comment ça s'est passé? (This literally means "How did it happen?" and is a good way to ask about the meeting.)
Remember: The best way to say "how did you meet" in French depends on the context and the people you are talking to.