We thank everyone who have donated, helped set up, etc.
Is "have" correct or "has"?
"We thank everyone who has donated" is correct. This is called subject-verb agreement.
The subject ("everyone") is singular, and "who" reflects the subject's number. Therefore the auxiliary verb "has" in the relative clause must be singular too.
Words such as either, neither, everyone, everybody, anyone, anybody, someone, none or each, are singular and need a singular verb.
As we just said, don’t be fooled if a singular subject is followed by plural nouns. For example, when you write “each of my daughters,” make sure the verb agrees with the singular subject each instead of the plural noun daughters.
And the singular subject “everyone who knows my daughters” should be followed by the singular predicate “is impressed by them,” not “are impressed by them.”