Sharepoint - Office 365 test user accounts

You could use an external user, which you don't have to pay for. An external user...:

• can use Office Web Apps for viewing and editing documents. If your plan includes Office Pro Plus, they will not have the licenses to install the desktop version of Office on their own computers.
• inherit the use rights of the Office 365 customer who is inviting them to collaborate on a site. That is, if an organization purchases an E3 Enterprise plan, and builds a site that uses enterprise features, the external user is granted rights to use and/or view the enterprise features within the site collection they are invited to.
• can perform tasks on a site consistent with the permission level that they are assigned. For example, if you add an external user to the Members group, they will have Edit permissions and they will be able to add, edit and delete lists; they will also be able to view, add, update and delete list items and documents.
• will be able to see other types of content on sites. For example, they can navigate to different subsites within the site collection to which they were invited. They will also be able to do things like view site feeds.

But External users can't...:

• create their own personal sites (what used to be referred to as My Sites). This means that they do not have their own SkyDrive Pro document library.
• see the company-wide newsfeed. They also cannot edit their own profile, change their photo, or see aggregated tasks. External users do not add quota to the overall tenant storage pool (this is determined by licensed users only).
• be an administrator for a site collection. However, you can designate an external user as a designer for your Public Website. This restriction also does not apply to scenarios where you have hired a partner to help you manage Office 365.
• By default, access the Search Center and will not be able to execute searches against “everything” (cross site collection search)

Reference: Understanding External Users in SharePoint Online.

Follow the steps mention in my other answer to enable external sharing.