Typically, SQL Server is able to successfully handle multiple people updating data at the same time. Occasionally, however, you may find that you run a query and the data that is returned is not quite right because someone else has simultaneously done something to the data you just queried.
There are 3 types of problems you can run into:
* Dirty data
* A "phantom read"
* A "non-repeatable read".
To solve for these, you can have a stored procedure execute with a "SNAPSHOT ISOLATION".
Here's a good read for further information: http://databases.about.com/od/sqlserver/a/isolationmodels.htm