LinkedIn Groups are free to set up. However, before starting your own group, we recommend that you look for an existing group on your topic. Recruiting group members is difficult and if someone has already done it for you, why not leverage those efforts?
LinkedIn Polls are another free tool. You can ask a question and supply up to five multiple choice answers. LinkedIn gives you a URL to the poll as well as an embed code for your blog or website. (An embed code allows you to put the asset from LinkedIn on your web page. Embed codes are also how people put YouTube videos on their websites or blogs.) You can create a string of poll questions, perhaps one per week, to create a small survey.
LinkedIn Events is a free place to promote your event. Once you set up the event on LinkedIn, people can find it through search and indicate if they will attend or follow the event. If a person is attending your event, it will show up in their activity stream so that everyone following that person is now exposed to the event.
LinkedIn InMail is a for fee service. Normally you must be “connected” to a person to send them a message. InMail allows you to bypass that restriction. You will work with the LinkedIn team to select recipients by title, group, location, etc. An InMail comes from a person, so pick a subject matter expert as the sender.