Forum Moderators: rogerd
Edit:
[phpbb.com...]
My bad, apparently phpbb groups can't be postcount-based. To size the effort for you, it would require some script that queried the db for users who've met your postcount criteria, and assign them to the group. This script could run entirely ooutside the forum and work daily, hourly, whatever. So it would be custom, but could live outside the phpbb entirely.
If you could live with manually adding the members to the group, it can be done with only out-of-box phpbb.