Here are some discussion forums where you can find other upholsters to discuss the various aspects of upholstery.
Upholstery Discussion Board - A discussion forum where amatuers and professionals can discuss all phases of upholstery. Most of the discussion goes on in one forum board.
Facebook has several upholstery groups:
Professional Upholstery & Soft Furnishings Group (This group has over 1200 members. It is directed more towards professional upholsterers.)
Upholsterer's friendly forum (This group has over 8500 members from all over the world. They are open to both professional and amatuer upholsterers.)
upholsterers and Craftsmen And craftswomen. (This group has over 2700 members who are either upholsters or other craftsmen or craftswomen.)