Our jira users are: internal001, internal002… external001, external002…
Now I want to count: every sprint, how many issues are assigned to internal users and external users.
You are almost there! The only thing: using function Count(), you are counting filtered assignees, not issues. To get the sum of sprint issues over those assignees, use the same filtering logic, only with Sum() aggregate and using [Measures].[Sprint issues at closing] as a numerical expression: