How do I invite people to join my social thread? Do I just send them a link that says they're invited or do I have to do more?
Go to your main Social Forum page.
Click on either "Social Forum Members" or "Social Forum Moderators". This will take you to the members section.
Below "Social Forum Moderators" to the right hand column will be something called "Invite Member" with a text box. Just type a member you want to invite and the members name will appear.
Alternatively, you can send a mass PM to members you want to invite (notice: you can only include 10 users into a PM) with a link to the social group and title of your group. They can click the "Join Group" button to automatically join the group on their own accord. To demonstrate, I did just that and I am now a member of your group "Smash 4 Conspiracies". I hope that helps.
Though it appears people can post in your Social Group without becoming members. That probably discourages membership. You can make it so only members can post in your Social Group by going to "Social Forum Tools"
Then go to "Edit Social Forum" which will take you to a window where you can edit the name and description of the social forum. Below that is a list of Social Forum statuses which dictates who can see and post in the forum.
It is possible that your Social Forum is Public meaning anyone can view and post
I suggest setting it to Member which means anyone can view the forum but only members can post in it. That encourages people to joining the group in order to participate in discussion instead of just waltzing it, posting, and then not joining. If you are looking for an increase in membership, I would change it to "Members".