You can currently tag contacts in Infusionsoft by using a meta field in the event editor for Event Espresso:
http://eventespresso.com/wiki/infusionsoft-integration/#ee3customizations
That method requires you specify the tag for each event.
You can use this code snippet to specify a global tag that will be used for all contacts. This is useful if you will be using a single tag instead of multiple tags.
First create a tag in Infusionsoft. There is a guide available here:
http://ug.infusionsoft.com/article/AA-00306/0/How-do-I-create-edit-or-delete-a-tag.html
Then locate the tag ID which will be a numeric number. You can find it by going to Infusionsoft --> Settings --> Tags. Get that number and then change 123 in the example code to your actual tag ID and save changes.
The sample code can be added to your child themes's functions.php file or a site specific plugin.
References: