Supervisors (subject, plural), especially those who are new to the field,
have (verb, plural) to attend orientation.
When you eliminate the intervening phrase, you’re left with:
Supervisors (subject, plural) have (verb, plural) to attend orientation.
Follow the Rules No Matter How It Sounds to Your Ear
Many people make subject/verb agreement errors because they mistakenly
trust their ear. But your ear may have been trained to hear something
as correct when it’s not. Be certain that your ear guides you correctly before
relying on “how it sounds.” Mistakes frequently occur when using specific
words that are always, by definition, either singular or plural.
For example, use a singular verb after each, everyone, everybody,
nobody, someone, every, one, another, and much. Use a plural verb
after few, many, others, and several.
Every supervisor is attending. (singular)
Everybody enjoys orientation. (singular)
Several supervisors are out of the office. (plural)
Many were interviewed, but few were able to answer. (plural)
Collective Nouns May Take a Singular or Plural Verb
A collective noun refers to a group. If the group is acting as one unit,
the noun is treated as singular. If the group’s members are acting individually,
the noun is treated as plural. (In British English, collective nouns
are treated as plurals.)
The Technical Review Board meets in May. (The subject is “Board,” one
unit, singular.)
The company is one of the largest employers in California. (The subject
is “company,” one unit, singular.)
My family are doing well. (The subject is “family,” the individuals are
emphasized, plural.)