Okay, I loaded the plugin and found the issue.  You will only see users that belong to groups you are the manager of.  This is controlled by the encoded query against the 'sys_user_grmember' table within the module link itself (which looks like this).

https://YOURINSTANCENAME.service-now.com/$ng_skills.do?sysparm_parentSkill=9b92ba407f232200c57212f44efa91ef&sysparm_parentDepartment=&recursiveSkills=true&recursiveDepartments=true&showLocation=true&tableName=sys_user_grmember&encodedQuery=group.manager=javascript:gs.getUserID()&fieldName=user

You can make yourself a manager of a group to see the users or you could change the encoded query in your module link if you want to open it up a bit.  For example, this would allow a user to manage skills for any user that is a member of a group.

https://dev45413.service-now.com/$ng_skills.do?sysparm_parentSkill=9b92ba407f232200c57212f44efa91ef&sysparm_parentDepartment=&recursiveSkills=true&recursiveDepartments=true&showLocation=true&tableName=sys_user_grmember&encodedQuery=&fieldName=user