Whatsapp just increase the maximum number of users allowed in a group from 100 people to 256 people. In 2004, this limit was increased from 50 users to 100 and has been increased again now because the app users keeps growing everyday.
As the most widely used instant messaging app and the most popular social messaging app, whatsapp really needs this change. Over 40 billion messages are exchanged on whatsapp daily and in jan 2016, Whatsapp announced that the $1 yearly subscription due has been canceled making the app completely free for use forever.
For those who have not been notified of this update, all you need do is upgrade to latest whatsapp version. This is the only way you can double the number of people allowed in one whatsapp group.
For the mean time, this expansion in group capacity is only available on iOs and android devices but Whatsapp has announced that it will extend the feature to other platforms as soon as possible.