answer:
When you start a new Meetup Group, it's important to pick the right topics: we'll use the topics you choose to match your Meetup Group up with folks who might be interested in joining. If you do a good job picking topics, your Meetup Group will be promoted to just the sorts of people who will want to become members of your Meetup Group.
Here's the trick: when you're starting a new Meetup Group, we'll do our best to recommend appropriate topics based on the name and description you've written for the Meetup Group. For this example, let's say we're starting a soccer Meetup Group:

On the page where it asks you to pick topics, the right column will be populated with topics that may match based on the name and description for the Meetup Group:

For this example, I'm going to choose 'Soccer' to start. Once I've done that, the right column will update with other related topics. I can continue to choose additional topics to add by clicking on boxes on the right side, or I can start typing in the fields on the left to choose new topics to add:

Some things to keep in mind:
For best results, think about what topics will bring people to your Meetup Group. What makes your Meetup Group special?
Overlapping topics are good -- potential members may have added 'pick-up soccer' or 'outdoor soccer' to their list of interests without adding plain old 'soccer', and you'll want to reach those people, too.
Choosing really broad topics like 'social' or 'new friends' will cast a wider net of potential members, but remember:
the broader the topic, the less likely any one person interested in that topic will actually join your Meetup Group. You're better off choosing good, applicable topics than looking for the most popular topics on Meetup.
That said, you do want to
use the suggested topics before creating a brand new one. Adding a brand new topic to the system means that no one in your area has expressed interest in that particular topic yet, and that means we'll have no one to notify about a new Meetup Group on that topic.
So, if your Meetup Group is an indoor soccer group for women, you'll want to add several separate topics: 'Soccer', 'Indoor Soccer', 'Women's fitness', etc... You can add up to 15 topics, so feel free to browse through the suggested topics to find ones that look good.
You can always edit your topics. If you already organize a Meetup Group, you can adjust the Group's topics any time you'd like. Head to your 'Group settings' under 'Group tools' and choose 'Topics'. You can get step by step directions for editing your Meetup Group's existing topics
here.