You can upgrade your plan here
You can purchase additional competitors for your current plan directly from your payment information section here. Simply click "Edit add-ons"
And then select "Add a competitor". You will then choose which social profile you would like to add your additional competitor(s) to. (which will bring you to the payment screen to complete your purchase.)
Each additional competitor is billed at $5 per month (or $45/addtional competitor yearly)