There are two ways to manually manage Geogroups, the first involves merging Geogroups to reduce the number of them and another involves applying material overrides directly to Geogroups.
Firstly, to merge Geogroups go to your browser and change the filter to show Models. Once there change the view to List. Here there will be a column named GGs, this stands for Geogroups. You can click on the category to sort the list by highest Geogroup count first.
On a model that has a high number of Geogroups, right click and show properties, from here, scroll down to Geometry Operators and expand the properties. At the bottom you will see Optimise Models, by clicking on the Settings of this property you will be able to select a number of optimisation options, one of which is Merge Geogroups, with this checked you can click Apply.
If there are several visuals with the same material override, you can instead go directly to the Geogroup by slecting the visual in the Developer Tab, right clicking and selecting View Properties. Under the Visual properties one is named Model, click on the name of the model in this property.
This will take you to the model in the library. Click the arrow to the left of the model name to show the Geogroups.
By selecting the Geogroup you can view its properties and apply you material to the Material Property, following this any model using this Geogroup will also use the material.