Add Call Apex Action to process step (1):

Select GlobalActionService (2) in "API Class Name" picklist, and select ExecuteActionGroup method (3):

Set groupId parameter value to Global Action Group Id (4):

To get the Global Action Group Id go to the Actions -> Action Groups, open the Action group you need and copy ID (5):

(Optional) populate "Source" parameter (6).

In the Source you can pass a JSON Array which contains sources for each action from your Action Group. There are 2 ways you can pass sources:

• Option 1: Pass Action ids with record Id for each action:
  "{Action1_ID}":"{record1_Id}", "{Action2_ID}":"{record2_Id}",... 
• Option 2: Pass Object Name and Record Id. All Actions which use specified object as a source will use specified record Id:
  "{Object1_API_Name}":"record1_Id",  "{Object2_API_Name}":"record2_Id",...

For example if there are 4 actions in the group, 3 of them use Account as Source object, and 1 uses Contact, and you pass: "Account":"{Account.Id}", "Contact":"{Contact.Id}", 3 actions will get the Account.Id as a source, and 1 will get Contact.Id.

(Optional) populate variables (7):