The Invitation to William was a letter sent by seven notable Englishmen, later named the Immortal Seven, to William III, Prince of Orange, received by him on 30 June 1688 (Julian calendar, 10 July Gregorian calendar). In England a Catholic male heir to the throne, James Francis Edward Stuart, had been born, and the letter asked William to force the ruling king, his uncle and father-in-law James II of England, by military intervention to make William's Protestant wife Mary, James's eldest daughter, heir, on the grounds that newborn Prince of Wales was allegedly an impostor.
The letter informed William that if he were to land in England with a small army, the signatories and their allies would rise up and support him. The Invitation briefly rehashed the grievances against King James, claimed that the King's son was supposititious (fraudulent) and that the English people generally believed him to be so, deplored that William had sent a letter to James congratulating him with the birth of his son and offered some brief strategy on the logistics of the proposed landing of troops. It was carried to William in The Hague by Rear Admiral Arthur Herbert (the later Lord Torrington) disguised as a common sailor, and identified by a secret code.
The invitation convinced William to carry out his existing plans to land with a large Dutch army, culminating in the Glorious Revolution during which James was deposed and replaced by William and Mary as joint rulers. William and Mary had already in April of that year, when William had started to assemble an invasion force, asked for such an invitation to be given, within the context of a secret correspondence since April 1687 between them and several leading English politicians, discussing how best to counter the pro-Catholic policies of James.
The signatories were:
The Earl of Danby http://en.wikipedia.org/wiki/Thomas_Osborne,_1st_Duke_of_Leeds
The Earl of Shrewsbury http://en.wikipedia.org/wiki/Charles_Talbot,_1st_Duke_of_Shrewsbury
The Earl of Devonshire http://en.wikipedia.org/wiki/William_Cavendish,_1st_Duke_of_Devonshire
The Viscount Lumley http://en.wikipedia.org/wiki/Richard_Lumley,_1st_Earl_of_Scarbrough
The Bishop of London (Henry Compton) http://en.wikipedia.org/wiki/Henry_Compton_(bishop)
Henry Sydney (who wrote the Invitation) http://en.wikipedia.org/wiki/Henry_Sydney,_1st_Earl_of_Romney
Of the seven, Danby and Compton were generally considered to be Tories, while the other five signatories were generally seen as Whigs.