can someone shed light on how group insurance through Employers/Alumni work?

we were looking for insurance through the regular routes, and i was quoted between $9000-11000+/yr, for two vehicles for me and my mom; this is without any at-fault accidents or tickets between the two of us

when i included my mother's School Board group insurance through RBC, we got a $5000 quote that's nearly $6000 in savings

how does that work? why are group insurance so much cheaper?