Sorry about your diagnosis. I really don't know of any signet ring cell exclusive support groups (honestly I haven't looked for any either). However, you are certainly not alone as you can see from my signature, and there are others on this site. It's been over a year since my diagnosis.
The good news is it is possible for you to get through this and get back to your normal life with just the normal colon cancer treatment regimen. If you use the search function in the quick links (up on the top left corner of the page) you can you can find stories of people with SRC including some long term survivors.
There really is not much uniquely that is medically recommended for SRC carcinomas except close monitoring because they grow fast and spread.
But to assuage your fears: descending/sigmoid colon cancer are supposed to be less aggressive than ascending colon cancers. Your cancer seems to have been caught early enough to not be too dire. You said you were otherwise healthy so you have the strength to get through chemo hopefully without complications.
It is really unhelpful to focus on the stats., talk to a cancer social worker to help with your anxiety if it gets overwhelming. You can get through this.
Diagnosed 4/8/16, age 29
Stage III, T4bN1 Tumor 7x6.5x2. Muscinous Adenocarcinoma with SRC features
2/16 lymph nodes
CEA 4/14/16 - 16.8
CEA 6/2/16 - 1.9
CEA 6/17/16 - 0.87, 7/16 - 1.33, 12/16 - 1.14, 4/17 - 0.6
FOLFOX began 6/24/16 - 11/25/16
10 round FOLFOX, 2 round 5-FU & Leucovorin
MRI & CT 8/16 - NED, CT 12/16 - 4/17 - NED