OG: How to import 2 related nodes from CSV files using Migrate module?
Okay so I'm assuming that you haven't used Migrate before. If you'd like some background reading take a look at the slides for the presentation I'll be giving at DrupalCon in Denver: http://denver2012.drupal.org/sites/default/files/slides/Migrate_0.pdf It should help you understand some of the concepts.

One thing that's important to point out is that you'll need two migrations: 


Using (a) and MigrateSourceCSV as a source and—assuming the groups will be nodes—MigrateDestinationNode as a destination. 
Using (b) and MigrateSourceCSV as a source and MigrateDestinationOGMembership (as you noted part of Migrate Extras) as a destination. 


The CSV files are actually really easy to use as a source. You can see an example of it in the Migrate docs: http://drupal.org/node/1152158

In your case you could setup the source for (a) with:

// The definition of the columns. Keys are integers, values are an 
// array of field name then description.
$columns = array(
  0 =&gt; array('gid', 'Id'),
  1 =&gt; array('group_title', 'Title'),
  2 =&gt; array('group_body', 'Body'),
  3 =&gt; array('group_admin', 'Admin'),
);

// Instantiate the class using the path to the CSV file and the columns.
$this-&gt;source = new MigrateSourceCSV('path/to/a.csv', $columns);


You'll also need to know something about your source when you setup the Map because it needs to create a field to store the source ids in... since gid's are basically nid's you can just cheat and use that source's schema definition:

$this-&gt;map = new MigrateSQLMap($this-&gt;machineName,
  MigrateDestinationNode::getKeySchema(),
  MigrateDestinationNode::getKeySchema()
);