If you add someone as a Friend, they will be able to private message you. Only your Friends and Ultimate members have the possibility to send you private messages.
The choice to accept or decline friend requests is on you. Moreover, you can also send friend requests to users provided that they meet at least one of the following criteria:
Added you to their Favorites.
Had a conversation with you (only possible if they're Ultimate members).
Were in your room.
This way we ensure that only members who have an interest in you become part of your friends, leading to a more loyal and stronger fan base. Note that users can choose to receive messages only from their Friends and Favorites.
Your friends are listed under "My Friends" tab in your profile, in the upper right corner.