-eer — one who is concerned with (affix)

engineer A2

/ˌɛnd͡ʒɪˈnɪ(ə)ɹ/ A person whose job is to design, build, or maintain machines, structures, or systems. M S

volunteer B1

/ˌvɑlənˈtɪɹ/ A person who offers to do something, often without being paid. M S

mountaineer C1

/ˌmaʊn.tɪnˈɪɹ/ A person who climbs mountains for sport or pleasure. M S

auctioneer C2+

/ˌɔkʃəˈnɪr/ a person who conducts sales by auction M S