Choosing Which LinkedIn Group(s) To Join

Due diligence comes first.
While a LinkedIn Group may look perfect for you, always do a little bit of extra research as it may save you a lot of aggravation later. Here are the things you should look for:
Group description. The description will give you clues as to what the purpose of the group is. Sometimes the description doesn’t match the group name.
Group rules. This will tell you how the group owner expects the group to behave. Whether the owner and his or her moderators actually enforce those rules is another matter altogether.
Group owner and managers. You can click on these people’s names, so do so. Find out who they work for. This will give you clues as to their motivation for being involved with this group. Groups owned and run by your competitors will probably not be glad to see you. Don’t invest your time in building visibility in a place where your welcome is uncertain.
And one that doesn’t mean as much as you think:
Number of members. This is a double edged sword. A large group will have more potential people to reach, but it will be more difficult to be heard above all the other people starting discussions. It is not uncommon for some of the really large groups to have hundreds of posts a day.
LinkedIn Groups have fallen on hard times the past couple of years. An ugly combination of group admin indifference, ham handed management by LinkedIn, and spammy users has resulted in a bad reputation for LinkedIn Groups. However, there are some absolute gems out there, typically groups where the owner is the main admin and heavily involved in the day to day discussions in the group. Two well run groups that I am a member of are “Step Into The Spotlight!” a group for marketing and branding and “Sales Playbook!” a group all about sales. The two owners have completely different styles but manage to run groups with multiple ongoing discussion threads.
These two groups are examples of what well run groups can be like. Unfortunately, they are the exception rather than the rule.