How do i get sites by group handle/name? How can I get sites by their group handle/name within twig templates?
Documentation only mentions querying sites by group id - getSitesByGroupId.
I would rather suggest you to do that in PHP but the Twig way would be the following

{% set allSites = craft.app.getSites().getAllSites() %}
{% set sitesWithGroup = [] %}
{% for site in allSites %}
    {% if site.getGroup().name == 'my group name' %}
        {% set sitesWithGroup = sitesWithGroup|merge([site]) %}
    {% endif %}
{% endfor %}