Hello Strangers,
Once you first make it to the Strangers Guild you'll have to go to the Leaders Room and go through some dialogue to be able to add any members to your party or to create any additional characters. Once you've gone through the dialogue just select "Register member", pick an empty spot, which seem to be represented by cards, press
to bring up another menu,
again to select "Register",
again at the information popup and then go through the character creation process just like you did when you created your main character. Once you finalize the character the trophy will unlock.
If there is anything I can do to make this solution better please don't hesitate to let me know.