Pressero has a CXML Level 1 PunchOut integration that allows for placing orders on a site, but the integration does not have any functionality for reassigning Site Groups to the users created on the site. Any users created through a CXML PO order will have the site user defaults as assigned through the site settings. Go to User Management > Site Users > gear icon button at the top of the list > Edit New User Defaults. The site groups that will be assigned to any new users are set under Group Membership.
For more information on CXML PO, please see the article cXML PunchOut