To invite other users to your Planio account, please navigate to Your Avatar -> Administration -> Users and click on the Invite users via email link on the top right corner. There you can enter the email addresses of the people you would like to add. They will each receive an email containing their personal login and password which allows them to log into your Planio account.
If you select the Also invite these users to existing projects checkbox, the newly created users will automatically be added to your existing projects allowing them to be assigned to tasks and to collaborate with you.
If required, you can also perform the steps (creating a user, adding them to the project) manually by clicking on the New user link and later editing the user and adding them to a project on the user's Projects tab.