Hi João,
Currently we’re not able to let the extra users to use the online services in an Office 365 trial subscription due to the trial license limitation.
In a trial subscription, Office 365 Small Business families get 10 user licenses per tenant, and the others have 25 user licenses. For the Office 365 Midsize Business trial subscription, we have 25 user licenses as the figure showed below. In a paid version of Office 365 Midsize Business, we can have up to 300 user licenses in a single tenant.
