Regular members and Post count groups can be used together, or separately.
If you give access to a board through regular members, you don't necessarily need to give the same access through post count groups at all.
If you don't give access to regular members, you can limit access to boards by post count - For example, have a board where everyone gets automatic access after they have made 100 posts.
The way to go depends on what you wish to achieve in the end.