The affirmative ustedes command of the verb pagar (to pay) is paguen.
To form the affirmative command in the ustedes form, you typically take the present subjunctive form of the verb. For pagar, the present subjunctive is pague for the singular form. When you want to express this command for a group (ustedes), you use the plural form, which is paguen.
So, if you want to tell a group to pay, you would say, ¡Paguen!