Google Inc. previewed a new communications service that combines features of email, instant-messaging and document-sharing to facilitate multiperson conversations online. The Google Wave service caps a years-long project to come up with a way to break down the barriers between different types of online communication services, said Lars Rasmussen, the Google engineer who led the development of the service along with his brother, Jens Rasmussen. The service, which won’t be open to the public for several months, will allow users to start a conversation, called a “wave,” and to invite their contacts to join it. Any member of the wave can put photos, notes or other content into the group, which updates in near-real time.