Learn Farsi: بعضی از افراد در این جامعه از فردگرایی حمایت می‌کنند. - Some of the people in this society support individualism.

It is built from the noun فرد (individual) and the suffix گرایی (-garâyi). This suffix is heavily used in Farsi to create abstract nouns that express ideologies, tendencies, or schools of thought, similar to the English suffix "-ism". Here, it turns "individual" into "individualism".

Why do we use the preposition از before فردگرایی when the English translation has no preposition?

The compound verb حمایت کردن (to support) specifically requires the preposition از (from) to connect to its object. So in Farsi, you don't support something directly; you literally "do support from" something.

Why doesn't the object of the sentence take the direct object marker را?

Since the verb حمایت کردن connects to its object using the preposition از, the object (فردگرایی) acts as an indirect object. The marker را is only ever used for specific direct objects that aren't introduced by a preposition.

Could I use مردم instead of افراد for "people" here?

You could, but افراد is a better fit for this context. مردم refers to "people" as an uncountable collective mass or the general public. افراد is the plural of فرد (individual) and refers to a countable group of individuals. Using افراد ties nicely into a sentence discussing individualism!