Brawl tiers have changed over time, Here will be all the previous tier groups.
1st tier group added on 7th Feb 2015 looked like this.
1-2, 3-5, 6-8, 9+
Tier 11 was the highest reached at this time.
Adjust to top tier +1 range 14th Feb 2015
1-2 3-5 6-7 8+
Change 28th june 2015
1-2, 3-4, 5-6, 7-9, 10+
Uncertain but may have exisred at some time.
The tiers are based on battle tiers of players in groups of 3. (note this is probably not accurate) It was said at one point to be this but it seems to have been reverted or progressed to another.
I think at some point they will be another range with 10-12 and 13+ included or 10-11 with 12+. This isn't official though. Current highest tier is 16 and there is around 6-10 in total there. A large amount in the next tiers down.
Note that these groups will change eventually when more tier up.