Calculating Length Of Service Categories Based On Start Dates

Ok, so I've been struggling with the logic behind this one for a while, so I thought I'd ask you guys for some ideas :)

Basically, I have the following table structure
Employee(employee_number, continuous_start_date, ...)

The problem lies in working out a summary of service categories (0-6months, 7-12months, 13-24, 25+).
I can work out the length of service in months with the following code

SELECT DateDiff(mm, continuous_start_date, GetDate()) AS 'Service in months'
FROM employee

So the first stage is to summarise the length of service into groups as mentioned above.

Then the final stage is working out how many people are in each group!

Hope I have given enough information - and please do not post a full solution - just some hints on how to get the desired result ;)

Thanks later, and in advance :p
-GeorgeV