New people should be added as a member in the "member" section by clicking the green button "add member".

You can add all the information that is needed (name, last name, e-mail address, language, ...) and link the new member to the right groups if this is relevant already.

Once your new member proves that he is committed to give input on the campaigns you're sending out, you can always send him/her a community invite (more information in the community invite help article).