Yes you can delete any groups except the default ones.
The auto groups extension allows you to set post limits for group membership
https://www.phpbb.com/customise/db/exte ... to_groups/
To add someone to a group...
users and groups > manage groups > members
This is better for adding multiple people to a group
manage users > enter their name > choose groups from the drop list on the right
You can then add that person to any groups on the list.