Google staffs SRE teams as either 8 in one location/TZ or two geographically distributed teams of 6 -- often some pairwise combination of U.S., Europe, and Australia to accommodate reasonable on-call shifts.

The on-call compensation varies depending on what tier of service they're offering. Tier 1 (5 minute response time) is 2/3 of your effectively hourly pay for on-call time outside of local business hours and 1/3 for tier 2 (30 min response time). Or time off in lieu.