There are two ways of sending invites to people. One is a general invite to the forum and the other is an invite to a specific topic.
Everyone who invites a member also gets the Promoter badge 
Invite people to the forum
You can do that by going to your user page and clicking following button.
Invite someone to a specific topic
At the bottom of ever topic, you will find the button for inviting people to that specific topic.
