I'm managing a small LinkedIn group: https://www.linkedin.com/groups/...
I would say that linkedin group are quite dead. Post from groups are rarely displayed on LinkedIn feeds. My group is mainly successful because it is the only one including "Phantombuster" in the name, so it is the first result if people look for it. That provide me few inbound lead for my consulting service. => if you target some keyword for which there is no group yet, go for it.
The most interesting feature of the linkedin group is the possibility to message all members. I made some small automation sending welcome message to new members and it startup some interesting discussions.
Create a company profile,Post actionable content, Use the Matched Audiences feature,Search for individuals,Leverage groups,Start your own groups,Use InMail,Follow the LinkedIn Marketing Blog,Use the paid LinkedIn Sales Solutions.