To make the process smoother, create a new group with the contacts that you wish to be removed from the other group(s). Advanced search is a valuable tool to help you find certain contacts if need be. More information on importing contacts can be found here
From here you can add conditions of First Name/Last Name etc and Send will pull all of the contacts that fit within that condition.
Once you've gathered all of the contacts into a new group, go to Contacts and click the Group tab and select the newly created group. Navigate to the Perform drop down menu and click Remove Contacts from Other Groups.
Next, select the group(s) that you want the contacts in your newly created group to be removed from, then click the Remove Contacts button.