Log in as the Admin user and under 'Cosmetri GMP Plan' (user menu, top right) you can subscribe additional user seats.
User seat subscriptions are tied to the renewal date of your main plan. For example, if your Cosmetri GMP annual plan subscription starts on Jan 1 and you subscribe 10 additional user seats on Feb 1, you will be billed 11/12th of the annual cost, with the additional user seat subscription then billed the following Jan 1 at the full annual price.
Please note, that a user seat subscription is specific to one application – therefore if you require a user to be able to access both applications, they will require two user seats, one in Product Manager and one in Cosmetri GMP.