function my_custom_activity_meta_stuff( $content, $user_id, $activity_id ) {
	bp_activity_update_meta( $activity_id, 'bpcat', $_POST['bpcat'] );
}
add_action( 'bp_activity_posted_update', 'my_custom_activity_meta_stuff', 10, 3 );


function add_bpcat_form_func(){
  $output.='
<div id="bpcat">
<select name="bpcat" id="bpcat">
  <option value="" disabled selected>Choose a Category</option>
  <option value="update">Update</option>
  <option value="news">News</option>
  <option value="video">Video</option>
  <option value="image">Photo</option>
  <option value="advertisement">Advertisement</option>
</select>
</div>
';
  echo $output;
}
add_action('bp_activity_post_form_options', 'add_bpcat_form_func');


function my_bpcat_func() {
$bpcat = bp_activity_get_meta( bp_get_activity_id(), 'bpcat' );
  echo '<span class="bpcat">Category: ' . $bpcat . '</span>';
}
add_action('bp_activity_entry_meta', 'my_bpcat_func' );