add send private message button in members directory below (You can actually add it anywhere)

send private message.php

/**
 * Get the User Id in the current context
 * @param int $user_id
 * @return int user_id
 */
function hibuddy_get_context_user_id($user_id=false){
 
if ( bp_is_my_profile() || !is_user_logged_in() )
 return false;
 if( !$user_id )
 $user_id = bp_get_member_user_id();//for members loop
 if( !$user_id && bp_is_user() ) //for user profile
 $user_id = bp_displayed_user_id();
 
 return apply_filters( 'hibuddy_get_context_user_id', $user_id );
}



/** Build the URL for sending private message**/

function hibuddy_get_send_private_message_url() {
 
$user_id = hibuddy_get_context_user_id();
 
if( !$user_id || $user_id == bp_loggedin_user_id() )
 return;
 
 if ( bp_is_my_profile() || !is_user_logged_in() )
 return false;
 
return apply_filters( 'hibuddy_get_send_private_message_url', wp_nonce_url( bp_loggedin_user_domain() . bp_get_messages_slug() . '/compose/?r=' . bp_core_get_username( $user_id ) ) );
}



/**Generate the Send Private Message Button**/

function hibuddy_get_send_private_message_button() {
 //get the user id to whom we are sending the message
 $user_id = hibuddy_get_context_user_id();
 
 //don't show the button if the user id is not present or the user id is same as logged in user id
 if( !$user_id || $user_id == bp_loggedin_user_id() )
 return;
$defaults = array(
 'id' => 'private_message-'.$user_id,
 'component' => 'messages',
 'must_be_logged_in' => true,
 'block_self' => true,
 'wrapper_id' => 'send-private-message-'.$user_id,
 'wrapper_class' =>'send-private-message',
 'link_href' => hibuddy_get_send_private_message_url(),
 'link_title' => __( 'Send a private message to this user.', 'buddypress' ),
 'link_text' => __( 'Private Message', 'buddypress' ),
 'link_class' => 'send-message',
 );
 
 
 /**The below function generates the html for the button and returns it. Let us build another function to echo that button. 
We are doing it for the sake of simplicity.**/
 $btn = bp_get_button( $defaults );
 
 return apply_filters( 'hibuddy_get_send_private_message_button', $btn );
}




/**Echoing the button**/

function hibuddy_send_private_message_button() {
 echo hibuddy_get_send_private_message_button();
}

/** 

Now, We have everything in place, we just need to hook it to  BuddyPress members 
directory items. BuddyPress themes normally provide the hook 
‘bp_directory_members_actions’ to which we can attach our generated button

Show the button in members directory
put this line in functions.php ( Do not put the code below to your bp-custom.php, 
that will generate a fatal error ) **/


if ( bp_is_active( 'messages' ) ){
 
add_action( 'bp_directory_members_actions', 'hibuddy_send_private_message_button', 30 );//experiment with the last value to change position
 }
 
 /** Source: 
 http://buddydev.com/buddypress/add-send-private-message-button-in-members-directory-on-a-buddypress-network/
 
 
 /** 
 Additional:
 
 You can put all the code to functions.php or bp-custom.php. 
 If you are putting the code to bp-custom.php, please avoid putting the code in step 5 there.
 
 As you see, the buttons looks a little bit overlapped. By using ‘send-private-message’ class in css, we can easily fix that.
Here is how I fixed for bp-default theme.


.item-list .send-private-message {
margin-top: 15px;
 
}
 
 
 **/