Respectful or polite equivalent of 誰でも. Common in announcements, invitations, and polite speech when referring to people without specifying who.
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どなたか means 'someone/anyone' in a question or request about a single unspecified person, whereas どなたでも means 'anyone at all, no matter who.'
Formed from どなた, a respectful interrogative for 'who,' plus でも, which gives an inclusive sense like 'even' or 'any.'