Possessives and Attributives

Q. I am proofing a training manual. It’s labeled “Participant’s Manual.” Shouldn’t it be “Participants’ Manual”? Thanks.

A. The author gets to decide. “Participant’s Manual” is for one person to use; “Participants’ Manual” is for two or more. You might compromise with “Participants Manual,” where the first word functions attributively.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]