all rows from left table with sum of amount from right table
Figured it out myself.  My where clause is telling sql to only show rows that are 'Bet' and have that date criteria, so without me realising it I'm telling sql to show me rows that had bets.

Here is my alternative query...

select i.accountnumber, i.firstname, i.lastname, sum(isnull(amount,0))
from accountinformation i 
left join accountactivity a 
    on i.accountnumber = a.accountnumber 
   and a.transactiondate &gt;='2015-07-31' 
   and a.transactiontype = 'Bet'
group by i.accountnumber, i.firstname, i.lastname


Putting the conditions in the join clause instead of the where clause makes the left join behave as expected.