The add members screen is used by guild leaders or member admins to selectively invite people to join the guild.
To invite someone to the guild you must know the name of the person you wish to invite and ideally should have told them to expect an invite so they do not consider it as unsolicited spam.
Type the name of the person you wish to invite into the box at the top, if a match is found click on them then click the Add button to invite them to the guild.
Please do not use this system to spam send out blind invites to people in the game as this will be considered SPAM and may lead to your account being banned.