A "legitimate" offer to have a company bought out is considered...material information....and it can be shared information to all within the company and outside of it who are considered Insider's and privy to that info....but they still cannot trade on that info...that's insider trading.

Regualtion FD of the SEC does not allow disemination of material information...such as a proposed buyout offer...to anyone not considered an insider...and if that does occur even if by accident it is required that the rest ofo the public ownership( common shareholders) be notifed also ...and I believe it is within a 24 hour period.