How to show the count of members that belong to a group?
I finally figured it out:


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The steps are: 


create a view for "content" type "group". Use "fields" and show "content:title", that's the name of your group
add relationship "Group: Node group" (required), then "(group) Organic groups group: OG membership" (required).
add filter "(OG membership) OG membership: Entity_type " and set to "is equal" to "user"
"use aggregation"
add field "(OG membership) OG membership: Entity id" and use "count distinct" option. That's your count of members.


If you want the count of nodes that are "linked" to a group you would just change the filter from "user" to "node".