I mention the power numbers because chances are the OP was putting down way more power than many of the riders in the group. I'm sure that the OP is much stronger than me, as are most riders around. On the other hand I can stay with a group and I'm somewhat competitive in Category 3 races. It's not about power in a group ride, it's about drafting.
On hills it's about power, yes, and weight, but you can fake hills if they're not too long, maybe a minute or less.
In the race below I averaged about 170-180 watts. My teammate Cliff (he appears at the beginning, middle, and end) averaged 380 watts. I got 2nd in the race. Cliff helped me do that but in the process he put down more than double my power.
Also, and it's worth noting. At the end of the clip, at around 8:23, when I am behind Cliff and he starts his leadout, my HR is 169 bpm. Cliff averaged 35 mph until he blows up.
During that time my heart rate dropped 5 bpm to 164 bpm. I was resting and recovering at 35 mph because I was drafting while Cliff led me through a pretty strong headwind.
That's the power of the draft. During that time I was doing just over 0 watts - I was doing some soft pedaling and coasting at 35 mph. This allowed me to recover before the sprint and I could do a better sprint than I otherwise could. I didn't win the race but Cliff's leadout allowed me to sweep up all but one rider from the break. Another rider in the break, in the orange/blue, would turn pro later as a cyclocross racer. In this race he was just a Cat 4. I'm sure he put down a lot of power - he's a super strong rider - but in this race the power of the draft worked against him. Individually he's really strong, in a group on a non-hilly course he's just fodder.