View Subscribe to a Topic
Put the following in a new template. Then link to this template, putting the topic_id the user is subscribing to in {segment_3}
{exp:user_groups:topic_subscribe topic_id="{segment_3}" return="sandbox/topic/url_title" refresh="0"}
{/exp:user_groups:topic_subscribe}
Visiting a template with this code in it does the same thing as checking the “Notify me of replies” checkbox when submitting a reply to a topic.