Hey there,
I am really sorry for the delay in this reply as it was a public holiday for us here.
Actually, the user profile and group page already contain a link for the about, you can check my attached screenshot below.
So I am not sure what kind of about us link you referring to?
Or are you referring to how to add a custom link on the user profile and group navigation bar?
If so, the file location you shared is correct, you need to use the following checking to determine whether the current page is user profile page or group page.
$customOption = $this->input->get('option', '', 'cmd');
$customView = $this->input->get('view', '', 'cmd');
$customLayout = $this->input->get('layout', '', 'cmd');
if ($customView === 'group' && $customLayout === 'item') {
// show a group about us link
}
if ($customView === 'profile') {
// show user profile about us link
}