Click 'Database Fields' in the 'Members' tabs on your management toolbar.
To add a new column/field in your database simply click on the 'Add Column' button on the left hand side and fill in the details. A new box should appear for each column. This also allows you to edit the columns. For more information on editing databse fields, see this article.
Adding fields to your membership form
Use the 'Who can see this column?' settings on the 'Edit Database Fields' page to decide whether to show the selected field in the membership form or not. 'Include in members' public profiles' decides whether the answers to this question are displayed in public profiles accessible from the 'Members' tab and forum posts.
Use GroupSpaces to manage your membership
Add custom fields to your database
Collecting information on members to populate database
Sending update requests
Editing member details