Can I make a plug-ins to view a certain group of people?

I’m not sure I understand what you’re asking.
The question is if you need a plugin to extract statistics from a group to see them?
In this case no, you do not need to write a new plugin if you want to collect statistical data you can use the Data Explorer plugin.
If you want to display them somewhere on Discourse you will need at least a theme, but most likely yes, you will also need a plugin.