Fastory let you invite quite simply invite your colleagues to a workspace, in order to collaborate with them on multiple stories.
On your workspace, just click on the "Invite Members" button, on top of the screen:
Then enter the email of the co-worker you need to invite, and chose his role. You can chose between Member & Admin. Members can edit everyone task, and Admin can edit everyone's task, and access billing and timeline settings.
Your colleague will receive an email, inviting him to accept the invitation to the workspace.