‘One less’ or ‘one fewer’?

Use "fewer" when referring to nouns that you can count, including "member." (My team has one member, two members, three members ...) That means either "one member fewer" or "one fewer member" is correct.

However, it's important to note that in casual, everyday English, many people will use "less" in this case. According to most linguists, that makes "less" acceptable -- which is different from "correct." When considering this sentence, you may want to think about who's talking and who's listening or reading. Using correct English is wonderful, but it's not always necessary!

BKlyn df's answer does not give the complete picture. It can be a matter of stylistic choice. From the Chicago Manual of Style (https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Usage/faq0304.html):

A. If the countable noun is plural, choose fewer; if it’s singular, choose less. (When CMOS says to reserve fewer for countable things, it’s talking about plural countable things. When it says to reserve less for mass nouns, it means singular mass nouns.) One is always singular: there is one less food group in the new pyramid; there is one less number in this column. Two (or more) is plural: there are two fewer food groups in the new pyramid; there are three fewer numbers in this column.