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: