[SalesForce] SOQL query on date field WHERE by year

Confirming @Eric, your query

List<CustomObj__c> list = [SELECT Name 
                           FROM CustomObj__c 
                           WHERE Expense__Date__c = THIS_MONTH];

is validated by the following:

• Date Format and Date Literals
• SOQL Date Functions

I tried out a version of your query myself, and it worked fine.

Though this is besides the point (the Date Literals work), there is always the option of "rolling your own" by something like the following:

Date startOfMonth = Date.today().toStartOfMonth();
Date startOfNextMonth = startOfMonth.addMonths(1);
List<CustomObj__c> list = [SELECT Name 
                           FROM CustomObj__c 
                           WHERE Expense__Date__c >= :startOfMonth
                           AND Expense_Date__c < :startOfNextMonth];

Try this:

list<Group> groups = [select id, name, (SELECT Name, Group__c, Business_Value__c, Meeting_Date__c, GPS_Meeting_Location__Latitude__s, GPS_Meeting_Location__Longitude__s FROM Meetings__r ORDER BY Meeting_Date__c DESC LIMIT 1
 ) from Group where Id =:BomaGroups];

Your limit 1 in the original query applied to the whole of the result set and therefore only returned 1 record. By using a sub-query you can get the records that relate to the groupIds you need. I have guessed at the lookup relationship api name being Meetings__r - this may need adjusting.