Out of the box it's not possible to copy permissions between site collections. You can copy permissions programmatically, as explained in http://mosswat.blogspot.com/2009/02/how-to-copy-permissions-between-site.html and http://social.msdn.microsoft.com/Forums/en/sharepointdevelopment/thread/25e54544-bf02-4861-af6d-a8a308c58b34.

However, it's considered a best practice to make a security group in Active Directory and add that security group to each of the site collections. Then all you need to do is assigning the group with same permission level.

Inspired by forum thread: http://social.technet.microsoft.com/Forums/en-US/sharepoint2010general/thread/566e88d5-53a6-4bd2-b3ca-05c384ea62eb

Please note

Also check out the SharePoint 2010 Best Practice Overview page at http://social.technet.microsoft.com/wiki/contents/articles/8666.sharepoint-2010-best-practices-en.aspx