The way you can assign competencies will depend on what combination of Totara products you are using. Assigning competencies by audience is typically only something you could do with Totara Perform enabled.
If your site uses Totara Perform, then you can find out how to assign users via an audience here:
https://www.youtube.com/watch?v=hraBgIqBl8o
https://www.youtube.com/watch?v=hraBgIqBl8o
You can also find more in our help documentation here: https://help.totaralearning.com/display/TH14/Competency+assignment
If you are not using Totara Perform or are on an earlier version of Totara, then you will need to assign competencies via a learning plan for users. You can find out how to do this here:
You can also find out more about competencies our competencies course, as well as in our performance management workspace.