Lookup values in report filter

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I want to create a report where I select an account from a drop-down and where RNOW displays the number of incidents closed by that account + the number of incidents closed by all accounts that are a member of the same account group.

For example I select "John Smith" who is a member of the group "Tier 1". My report should show:

Period	John Smith	Tier 1
2016-Q1	150	1502
2016-Q2	165	1726
2016-Q3	162	1645
2016-Q4	149	1662

 

But I'm stuck defining a filter where acct_groups.group_id must get the same value as the acct_groups.group_id for the selected account John Smith (which is stored in a variable $Agent).