。
The famous diplomat gladly joined the committee.
Literal
Famous-[attributive-な] that diplomat [topic-は] gladly committee [to-に] participate-[inward-benefactive-してくれた].
外交官 ('diplomat') is built from 外交 (foreign relations) + 官 (official). 快く ('gladly, willingly') is the formal adverbial form of 快い ('pleasant, willing'), often used in written contexts to describe someone agreeing or participating with grace. The inward benefactive ~してくれた marks the participation as a favor done to the speaker's group.