If you have a LinkedIn account you have more than likely received a large number of invitation from of a remote aquiantence asking you to join their group. If not, they will be there soon.
It seems there are at least a few so called…and questionably certified… LinkedIn experts who highly recommend that you invite your entire contact list to join your LinkedIn group via mass email to that list. By definition that effort would be considered Spam, especially my those who or not interested or do not know the person well.
LinkedIn Groups are excellent forums for discussions. Unfortunately, many misguided experts are now telling business people that they are even better to promote a product or service. This is a huge Fail.
What I have found what works best is to Blog about the group or at the very least post a status update on LinkedIn and Google+. I have found that by placing info about the group in the form of content that is searchable is ten times better than Spamming several hundred people in an email. Letting someone find the groups they are interested in works the best.
Use the power of the search engines to help bring the people you WANT in the group. The days of just gathering up as many bodies as possible in hopes several of them like what you say are WAY over.
Get social people. It really works better than Spam.