You may have imported all your contacts but you now wish to create some new groups based around your contact custom fields...
1. Create a custom report to filter out an option within your custom field.
2. Wait until you are notified by email that your report has completed.
3. Return to Reporting and on the line of your report you can see the "create as group" hyperlink which you click on.
4. Name and save your new group.
5. Repeat this process for each of your options and/or each of your custom fields.
If you have further questions about this please raise a support ticket.