So here is the solution to my own question.
<permission-group
android:name="bla.bla.groupPerm"
android:description="@string/perm_get_desc"
android:icon="@drawable/ic_launcher"
android:label="@string/perm_label" />
<permission
android:name="bla.bla.GET_PHOTOS"
android:description="@string/perm_get_desc"
android:label="Access photos"
android:permissionGroup="bla.bla.groupPerm"
android:protectionLevel="dangerous" />
<permission
android:name="bla.bla.DELETE_PHOTOS"
android:description="@string/perm_delete_desc"
android:label="Delete photos"
android:permissionGroup="bla.bla.groupPerm"
android:protectionLevel="dangerous" />
Apparently, the permission group name should contain the package name also and not just the permission name, as in my previous example: groupPerm. I.e., the correct name should be: bla.bla.groupPerm, where bla.bla is the namespace. If you don't do this, the permissions will show under Default group and would be listed: bla.bla.DELETE_PHOTOS and bla.bla.GET_PHOTOS.