Learn Italian: Ciascuno dei partecipanti ha un ruolo. - Each of the participants has a role.

Both ciascuno and ognuno mean "each one" or "everyone" and are often interchangeable. However, ciascuno is usually preferred when emphasizing the individual members within a specific group, like "each single one of the participants." That said, ognuno dei partecipanti would also be grammatically correct here.

Since partecipanti is plural, why is the verb singular (ha)?

The subject of the sentence is ciascuno (each one), which is singular. Just like in English ("Each of the participants has..."), indefinite pronouns like ciascuno and ognuno always take a third-person singular verb, even when they refer to a larger group.

Would ciascuno change if the participants were all female?

Yes! Unlike the English word "each", ciascuno must agree in gender with the noun it refers to. If the group consisted entirely of women, you would use the feminine form: Ciascuna delle partecipanti. The masculine ciascuno used here implies a male or mixed-gender group.

Just to refresh, why is it dei partecipanti and not degli partecipanti?