Firstly, you should NEVER insert SharePoint data directly in the content database.
Secondly, you SHOULD use the SharePoint Object Model to insert items into Lists.
Also, your question sounds a bit strange. You are saying you want to insert data into a SharePoint Field which is of type Multiple Users (Multivalue field). The SharePoint User Field takes exactly that... Users. Before inserting these Users (I take it you have Active DIrectory Usernames) you would have to create User Objects (Instantiate the SPUser Object using it's constructor) and pass these User objects into the Multivalue field.
THe code sample for the above can be found here: http://exxlence.com/2009/02/08/simplify-getting-and-setting-fields-of-type-spfielduser-spfieldurl-and-spfieldlookup-using-extension-methods/
The important part on that page is this part where you Set the Multivalued User/Person field:
public static void SetFieldValueUser(this SPListItem item,
string fieldName, IEnumerable principals)
if (item != null)
SPFieldUserValueCollection fieldValues =
foreach (SPPrincipal principal in principals)
item.Web, principal.ID, principal.Name));
item[fieldName] = fieldValues;