LinkedIn allows you to send group invitations from the Groups page because the folks at LinkedIn feel that invitations should come from the group owner — namely, you. Here are a few do's and don'ts to keep in mind as you craft your invitation:
Do relate the purpose and benefits of the group. People are busy and need to understand why they should join this group. Explain the benefits of being connected to other people, the ability for professional development or advancement, and what you hope to accomplish with this group. Remember, you're sending this to LinkedIn members, so don't worry about explaining LinkedIn — just explain your group.
Don't go on forever. One to two paragraphs is the maximum this invitation should be. Introduce yourself, introduce the group name, give people the benefits of joining, encourage them to join, include the link to the LinkedIn group page, and sign off. No one will read a long diatribe or laundry list of reasons to join. Use bullet points and short sentences whenever possible.
Don't put other offers in the e-mail. Some people use this as an opportunity not only to encourage folks to join one group, but to push a second group invitation, or highlight a link to the group's non-LinkedIn website. The moment you start presenting multiple options for people, you lose their attention, and they won't sign up.
You can use any e-mail program to create an invitation to your group, or you can go to the Manage link of your group and click Send Invitations from the navigation options on the left-hand side of the screen. You simply type in the name of your first-degree connections (separated by commas), a Subject line, and a Welcome Message, which can look as simple as this:
Hello,
You are hereby invited to join the new So Cal Comic Con Group on LinkedIn. Joining this group will allow you to find and contact other people interested in the convention, so you can stay in touch, gain referrals, and view other attendees’ LinkedIn profiles.
Hope to see you in the group!
–Joel Elad, Co-Promoter, So Cal Comic Con
After you send out the invitations, as members respond, they're moved from the pre-approved list to the current list of your group, and the small group logo appears on their profiles.