In Act you create a group. Then add contacts to the group. You can send emails to defined groups or use the group to search by.
For example, I go to a tradeshow and capture contact leads. When I return to the office I create a group (MS CRM calls them marketing list) and add the contacts to the group. I can now easily pull up a lookup (view) by the groups.
Sounds like Hubspot wants to use a custom and predifned field to place a common value for all contacts that we want to group on them search that field for the value later.