A group's owner can invite other people by using Jive SBS to send them an email. Go to the group's main page. In the Actions list, click Invite people to join this group. In the form you get, enter the email addresses of people you want to invite and feel free to edit the message however you like. Keep in mind that if you're using a private version of Jive SBS, people who aren't already using it might not be able to join your group.
If people you want to invite are in your email address book, you can import their contact information. First, export contact information from the email application as a CSV file. After you've exported, on the Invite People... page, click the Import Contacts link to import the information into Jive SBS.
Depending on how the group's set up, the group's owner might need to approve new members.