Correct spelling, explanation: attendant is a noun derived from the verb to attend, meaning a person who attends to something/somebody. As a rule, many nouns describing people and functions end in the suffix -ant (servant, confidant), and attendant is no exception.

Definition of attendant:
Noun
1. A person who takes part (attends) in an occasion/situation
The wedding attendants were all expected to wear elegantly.
2. A person who provides services, usually in the form of taking care of something or someone
The flight attendant was very nice and brought a blanket to my seat.
Adjective
1. accompanying (about a person, occurrence, effect, etc.)
He didn’t know much about the city and its attendant tourist attractions yet.

Incorrect spelling, explanation: technically this could also be a correct form, as the suffix -ent functions similarly to the suffix -ant, but attendent is not a correct form. Be careful with suffixes, even when they seem like they have the same meaning. The suffix -ant is most likely there because the word is borrowed from French attendant and the form did not change after the borrowing.