We do not allow non-admins to upload contacts and potentially pollute your contacts DB. However, it's possible for people to add themselves to your contacts DB.
They can do so by clicking on the 'follow' button on your Tikkl pages. This will ask the user to Log in/ Sign up to follow. Once they login and follow, a 'Follower' tag will be added to the contact and you can filter by using this tag.
You get more information about the interest areas of your followers when they click on the follow button by setting up interest areas in your org settings' profile page.
Each interest area includes a tag which will be added to the contacts with a prefix @INTEREST@. You can use this to filter people by interest when you send out invitations to the various activities of your group.