Follow step by step
1. class-fep-cpt.php change https://github.com/shamim2883/front-end-pm/commit/61f11cb4f402dacfb657b7293c5320e92dc3fb64

in clas-fep-group-message.php change following
2. change add_filter( 'fep_filter_display_participants', array($this, 'display_participants' ), 10, 3 ); to add_filter( 'fep_filter_display_participants', array($this, 'display_participants' ) );
3. add add_filter( 'fep_filter_cpt_display_participants', array($this, 'display_participants' ) ); bellow previous line
4. change if( ! $participants && $group = get_post_meta( $postid, '_fep_group', true ) ){ to if( $group = get_post_meta( $postid, '_fep_group', true ) ){
5. change function display_participants( $out, $par, $participants ){ to function display_participants( $out ){
6. change if( ! $participants && $group = get_post_meta( get_the_ID(), '_fep_group', true ) ){ to if( $group = get_post_meta( get_the_ID(), '_fep_group', true ) ){