Each WikiTree member has their own account but there is only one tree here. As you've indicated, you can use the New Person option from the Add drop-down menu to add an unrelated person.
For living individuals, if they'd like to share their DNA test info here, they must register at WikiTree (at least as Family Member) and enter their own DNA test info. They can make you a manager of their profile, if desired. If a profile already exists, you can add their email address and invite them to join.
For deceased individuals, if you manage their DNA test info and their profile here, you can enter their DNA test info on their profile.
See the DNA FAQ for more info.