SQL Query Error while using Group by clause

Question

What i am trying to achieve is that i want result set returned in two groups for two username with latest data

SELECT TOP(100) id , insertdate, modifieddate, detail, username FROM MyTable

WHERE username = 'AAA' or username='1234'

GROUP BY username

ORDER BY insertdate DESC

But when i run this query i get error :

Column 'MyTable.id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

How do i achieve this?