If you're not picky about having the seconds appearing in the grouped capturetime output, you can accomplish this with the below. If you need the seconds you can add them back onto the end of the grouping substring.

select convert(varchar(20),capturetime), SUM(snapshotvalue)
from snapshotdata
group by convert(varchar(20),capturetime)

Alternatively you can play around with the datetime conversion to varchar options that are documented here:

http://msdn.microsoft.com/en-us/library/ms187928.aspx

I'm a little confused by the 3rd example there, is this supposed to be 1325? Based off the requirements of capturing the sum of the snapshotvalues, the below query should get what you're after.

select dateadd(minute,1,convert(varchar(20),capturetime)), sum(snapshotvalue)
from snapshotdata sd
group by dateadd(minute,1,convert(varchar(20),capturetime))