In addition to FranksDrebin’s post, here’s the forum post by Scott Mercer regarding groups and matchmaking.
In this thread, Scott made a list on how your SR is influenced:
- What was the quality of the enemy team and your predicted win chance? You gain more on a win if your predicted win chance was <50%. You gain less on a win if your predicted win chance was >50%.
- Are you playing consistently? New accounts or accounts that have been inactive will see larger magnitude changes both upwards and downwards. This settles back to normal as you play additional games.
- Did your 3000+ SR recently decay due to inactivity? If this happened, you’ll gain more SR on a win until you get back to your “undecayed” SR.
- Are you a Platinum-tier player or below? If you performed particularly well or worse than what is considered a typical performance during a match, then there’s a small SR modification to reflect that.
- Is your current SR really high? Your SR increases less on a win than it decreases on a loss as your SR approaches the systemic limit of 5000.
Also, I noticed that this thread has 666 likes 
