を中心に: a central focus and the surrounding scope
Identify a person, place, or topic at the center of an activity. Explore a focus versus an exclusive limit, physical centers versus target groups, and noun-modifying forms.
Who is the main target?
学生を中心に、意見を集めました。 means that opinions were gathered primarily from students. Their views are central, but the sentence does not establish that nobody outside that group was asked.
地域の人を中心に、清掃活動を始めました。 describes starting cleanup activities with local people at the center. Even with a person noun, the following verb helps distinguish respondents in a survey from members of an activity.
A central location or topic
駅を中心に、店が増えています。 says that shops are increasing around the station as a center. They need not all be inside the station; the expression can describe a surrounding area.
今日は安全対策を中心に話します。 makes safety measures the main topic of today's talk. It gives the explanation a focus rather than requiring every sentence to mention that topic exclusively.
Different from だけ, and usable before nouns
学生だけに聞きました。 limits the people asked to students. 学生を中心に聞きました。 makes them the primary group, so the survey scope is not necessarily identical.
駅を中心とした地域の地図です。 uses を中心とした to modify 地域, an area centered on the station. 新しい制度をめぐる議論 instead means discussion surrounding a new system; distinguish a spatial center from discussion involving different positions.
Quick check: Does 初心者を中心に募集します。 necessarily prevent experienced people from applying?
Show the answer and why
Not necessarily. Beginners are the main recruitment target. The detailed eligibility requirements determine whether experienced people can apply.