Hack to enable invitations for Trust level 1 users

Support
11 
after_initialize do
  Guardian.class_eval do
    def can_invite_to_forum?(groups=nil)
      authenticated? &&
      (SiteSetting.max_invites_per_day.to_i > 0 || is_staff?) &&
      !SiteSetting.enable_sso &&
      SiteSetting.enable_local_logins &&
      (
        (!SiteSetting.must_approve_users? && @user.has_trust_level?(TrustLevel[1])) ||
       is_staff?
      ) &&
      (groups.blank? || is_admin?)
    end
  end
end

I guess this is the code you need. Put it in plugin.rb file. Since I am not in PC I didn’t tested it.