Is Google Inbox already habilitated for Google for Work (Apps) domains which are under Rapid Release cycle? I have received an invitation, but I can't seem to get into the Inbox site.
Google Inbox is not available for Google Apps accounts. They have not made any announcements about when that might change.
Enable Inbox by Gmail
You may receive requests from your users for access to Inbox by Gmail. Inbox by Gmail is a new product brought to you by the Gmail Team. You can make Inbox by Gmail available for your invited users, but it's not meant to replace Gmail. Learn more about Inbox by Gmail.
Only domains who receive an invitation may enable Inbox by Gmail. In all cases the domain administrator will decide whether or not to enable it for their domain or organizational unit. To request an invitation to Inbox by Gmail for your Google Apps for Work domain, please send an email to email@example.com.