Create a path to node/add in twig
According to According to Functions - In Twig Templates just Try 

{{ path('node.add', {'node_type' : 'yourcontenttypename'}) }}


as the example for article content type

{{ path('node.add', {'node_type' : 'article'}) }}


Update
Try 

{{ path('entity.group.add_form', {'group_type' : 'yourgrouptype' }) }}