I am new to Episerver. I'd like to create a visitor group based on a specific utm parameter, is this possible?
It depends on your need. How you would like to use them?
Are you expecting the Visitor Group Criteria to take input for UTM params and then create personalized content based on that Criteria?
It will be helpful if you can describe your requirements in more details.
Yes, you can create a custom visitor group. You just need to follow the standard implementation approach and define by Episerver and then write down your custom logic.
While you could write the criterion yourself as Ravindra suggests, assuming you're looking to create a visitor group based on a "utm_source" or similar querystring parameter then there's a ready-built querystring criterion available in the Episerver nuget feed: