As an admin and you can add Door Access several different ways.
1. Go to Users > filter on the User you are trying to grant Door access to > click "Edit" > click "Door Access" > click "+ Assign Group" button > select the appropriate Access Group for that User by clicking on the "Assign Group" button.
2. Go to Users > filter on the User > checkbox the box by their name > click "Select Actions" > click "Add selected Users to Group" > select the group that they need to be in.
3. Go to Access Groups > click on the Access Group that you are trying to add the User to > click "+Add User to Group" > in the search box start typing their name and then when they populate you would checkbox their name to select them and then click "Add User".