I find credits (not coins) are affected by your activity on the site: uploads, downloads, creating bundles, sharing links, rating posts (you have 20 free gold daily to give away) and so on.
The fastest way to level up (user group) to me is to:
* Choose a designer, search for her/his designs
* Buy the patterns you dont have, upload the ones that are not yet in the site
* report any issues you encounter: duplicate uploads, wrong titles... (use this thread for reporting issues: http://www.pindiy.com//forum.php?mod=viewthread&tid=217867