BuddyPress.org

@enrige,

not sure what you need. This button IS near Join Group !

in bp-default theme, are 2 reference to “New Topic”

/forums/index.p:23 (showing a link under Forum Directory title)
/groups/single/forum.php:19 (showing above topics list)

And in bp-default/functions.php: 132 you have the do_action code who shows the button beside the Join Group.
add_action( ‘bp_group_header_actions’, ‘bp_group_new_topic_button’ );

If your theme doesn’t use bp-default functions.php, put this do_action in your theme’s functions file