In short: There are many methods to pool forecasts. The most commonly used is the arithmetic mean of probabilities. However, there are empirical and theoretical reasons to prefer the geometric mean of the odds instead. This is particularly important when some of the predictions have extreme values. Therefore, I recommend defaulting to the geometric mean of odds to aggregate probabilities.
I wrote previously about this topic in LessWrong, and my thoughts have matured quite a bit since then. If you are interested in forecasting, I think you will find this an interesting read. Read more in this link.