Add new group members via one of the three options at the top of the page (add new group member; import group member; invite group member)
Once the group member is added, you will see the list of group member start to populate.
To allocate the registration types you have pre-purchased,
Click on the action drop down list at the bottom of the page and select what you want to allocate out
Check the boxes at the front of the group members in your list and click on start.
This will then allocate the registration that you have selected out to the members you have selected and add a green tick mark under the registration table.