Partner-enabled accounts can create sub-accounts, which allow other users to use their AWS credentials.
Go to Settings -> Sub-accounts and click the New button.

Provide a name and click Save.
The next step is to adjust the settings of their account including sharing groups, permissions, etc. Once you have configured of the account's preferences, click the Operate button to activate the sub-account.

To invite users to a sub-account, click the Invitations tab and enter the email addresses of the invited users. Be sure to choose the appropriate permissions that you want to grant each user.

The Operate button is intended as a management tool for a RightScale Partner that is setting up sub-accounts for their own users.
Click the operate button for temporary access (Default: 2 hours) to a sub-account, which can be used as a useful troubleshooting tool. For example, if a sub-account user contacts you with a problem such as, "My server, 'FunSite FrontEnd' keeps giving me an error when I try to launch it," you could click the Operate button to gain temporary access to their account to inspect their deployments, templates, etc.