Using Validation Groups Inside ASP.NET User Controls

Validation groups allow you to validate data entry controls in groups. Server controls such as validation controls, Button and TextBox have ValidationGroup property that takes a string value. All the server controls having the same ValidationGroup value act as one validation group. Validation groups come handy in situations where you wish to validate only a small set of controls from many controls housed on a Web Form. Using validation groups is quite easy and straightforward. However, if you have a validation group inside a user control and there are more than one user control instances on a Web Form you face some problem.

http://www.binaryintellect.net/articles/13427d3d-1f98-4dc0-849b-72e95b8b66a2.aspx