Earth is where Cole, Jenna , Dalton, Brady, and Joe come from. They enter the Outskirts through a Wayminder's wayport: Dalton and Jenna kidnapped for slavery, and Cole coming through willingly to rescue them. Joe and Brady enter the Outskirts at separate times before the series: Joe accidentally walks into a wayport by a beach, and Brady claims to have entered a dream. It is not mentioned in the series exactly how Brady entered the Outskirts.
Kidnapping from Earth was illegal until Stafford Pemberton came to the throne and passed a new law.