Are you talking about members of the website or users of Umbraco? Assuming users of Umbraco, you can use the UserService to hook into events and respond to them appropriately (e.g., by storing info to your main DB):

Umbraco.Core.Services.UserService.SavedUser += UserService_SavedUser;

If you are actually talking about members of the website, you can do that too:

Umbraco.Core.Services.MemberService.Saved += MemberService_Saved;

You typically do event subscription like that shown above as described here: https://our.umbraco.org/Documentation/Getting-Started/Code/Subscribing-To-Events/

public class LogWhenPublished : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        Umbraco.Core.Services.ContentService.Published += ContentService_Published;
    }
}

That is the example they show at the above link. You essentially create a class that inherits from ApplicationEventHandler, then override the ApplicationStarted method to add your event subscription code (in your cases, you'd listen to either user or member events).

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