"Who" refers to the entire group. Fred Hall was a member of the group.
We could write a similar sentence like this:
"Fred Hall was one of the group of twenty who set up studios there in the same year."
We could also write:
"A group of people set up studios there in the same year, and Fred Hall was among them."
If someone asked "Who set up studios that year?" the answer would be a list of peoples' names. "Fred Hall" would be one of the names on the list.
The writer of the sentence chose that word order because he liked the flow. He liked the way it led into the next sentence. This word order slightly emphasizes the group of people and the studios. When we read this sentence, we expect that the next sentences will not be specifically about Fred Hall, but about the whole group and about the studios.
I did a Google search for the sentence, and I find that the context is:
"Fred Hall was also amongst the group who set up studios there in the same year. Norman Garstin arrived the following year and before long, an artist colony was firmly established. Many of the residents had the opportunity to renew friendships developed in similar colonies in France during the preceding years."
So, I was right. Notice how the sentence leads us from the subject of Fred Hall, to the subject of the artist colony that was forming. That's why this word order was chosen, rather than one that might have been simpler or easier to understand.