You must first have an API key for a valid meetup.com member account (see Developers section on meetup.com).
Note: If the meetup group is private then your account must also be a member of that group. If you're not a member of a private groups you cannot display events from that group.
To find the ID for a group, make a groups API call and look for the id value inside the item.
If the group's url is http://joomla.meetup.com/39 then put http://api.meetup.com/groups.xml/?topic=joomla&groupnum=39&key=[yourkeygoeshere] into a browser address bar.
Or if the group's url is http://www.meetup.com/londonhumanists then put http://api.meetup.com/groups.xml/?group_urlname=londonhumanists&key=[yourkeygoeshere] into a browser address bar.
One of the methods above will return the ID needed for the module settings.