1. The people on the line takes the life jacket one boarding the boat

2. Few people know how to waterski
3. One of our friends brings gifts for the Harper’s party
4. All of the musicians is invited to perform for the talent show

To determine which sentences have grammatical errors, we need to examine the subject-verb agreement in each statement.

1. The people on the line **takes** the life jacket one boarding the boat.
The error in this sentence is the verb "takes," which should be "take" to agree with the plural subject "people." To correct this sentence, it should say: "The people on the line **take** the life jacket when boarding the boat."

2. **Few people know** how to waterski.
This sentence is grammatically correct. "Few people" is a plural subject, and the verb "know" agrees with it.

3. One of our friends **brings** gifts for the Harper’s party.
This sentence is grammatically correct. "One of our friends" is a singular subject, and the verb "brings" agrees with it.

4. All of the musicians **is** invited to perform for the talent show.
The error in this sentence is the verb "is," which should be "are" to agree with the plural subject "musicians." To correct this sentence, it should say: "All of the musicians **are** invited to perform for the talent show."