1. "All staff are invited." This is the most grammatically correct and formal option, emphasizing that all staff members are invited.
2. "All staff invited." This is a more informal option, often used in titles or headings. It implies that the invitation is addressed to all staff members.
3. "All staff have been invited." This option emphasizes that the invitations have already been sent.
4. "All staff were invited." This option emphasizes that the invitations were sent in the past.
5. "All staff is invited." This is incorrect grammar and should not be used.
Ultimately, the best option will depend on the specific context of your sentence.