You can say you’re friendly on a resume by framing your friendliness as a soft skill. For instance, you can list interpersonal skills or people skills in your resume’s skills section.
To prove that you’re a friendly person who’s good with customers and easy to work with, provide specific examples of how your friendliness has helped you be successful at work in the past. For example, you could write a bullet point stating that you had a 99%+ customer satisfaction rate in your last job as a customer service representative.