Is is possible to display only the members of the group of the logged in user under the Members tab? It currently shows all members.
If not, then how can i remove the members tab completely?
wpForo members tab cannot show only online members, so to disable the tab you can restrict users by their usergroup, Uncheck Front - Can view members.
For online members, you can try wpForo online member's widget, from Dashboard > Appearance > Widgets.