So I did the content sharing subscription and I wanted to know how this worked? Are they able to see my private dm notes on the screen? How do I share my campaign and purchases with them? 

Players can't see DM notes. Content sharing allows members of the campaign to have access to any content purchased by other members.

In order for players to join, you need to send them the join link on the campaign page.

1. Share the Campaign Invite Link

2. Check the 'Enable Sharing in this Campaign' slider

All purchased content by anyone in the campaign will be shared with others in said campaign.