Suzan,
This is very doable.
1) Make a report based on the Client table, and have it include a subreport based on the Event table. Join the main and subreports on the ContactID fields
2) Make an unbound form that has a combobox on it, and use a SQL statement like this for the ControlSource:
SELECT DISTINCT CompanyName FROM Client ORDER BY CompanyName
3) Make a commandbutton on that form, and use code like this in its Click event:
Code:
DoCmd.OpenReport ReportName:="YourReport", _
WhereCondition:="[CompanyName] = '" & Me!ComboBoxName & "'"