Support for a public domain program? No way, they say! Actually, as a user of Linux, you probably know better.
GRASS started by having a GRASS Information Office at USA/CERL. There were also very active users outside USA/CERL, who provided valuable user support. GRASS had annual users' meetings, listservers for users and developers, etc. Companies provided value added support services on a contractual or fee basis.
Various people have developed valuable books and training materials on GRASS. Several universities used to conduct training courses in GRASS. I don't know how many of these are continuing. If training courses interest you, try asking on the usenet newsgroup comp.infosystems.gis (see below for more on this newsgroup).
Valuable "books" available on the Internet are noted in the References (Section 10).
World Wide Web-based training materials, including training in GRASS, are highlighted in the CyberInstute Short Course in GIS (then scan down for the link(s) to Web-based tutorials in GIS).
One of the better GRASS tutorials is Project Assist's - Intro to GRASS
Other good sites:
Starting the hunt for mostly free spatial data by Stephan Pollard This is based at the Center for Advanced Spatial Technology of the University of Arkansas, another early educator with GRASS.
After getting trained by the books and Web-based tutorials noted just above, where do you turn to for specific advice???
Probably the best source of support these days is usenet newsgroup comp.infosystems.gis If you're not familiar with newsgroups, ask your network administrator or Internet service provider. comp.infosystems.gis contains modestly heavy traffic on such topics as
"how do I find data on this topic for this area?"
"how do I convert these data for use in my Aardvark GIS?"
"how do I get this function to work in my Aardvark GIS?"
"which GIS can I use to solve my particular problem?"
GRASS used to be one of the top GISs discussed on this group. Traffic in GRASS is dropping slightly, as its user community matures. However, there are usually answers to your questions, if you post them. You might also do a "power search" on subject:GRASS [& your own subject of interest here?] and newsgroup:comp.infosystems.gis in DejaNews to see what might appear from the usenet archives.