Fetch user details based on user Alias/ Display Name and Update it to People or Group Field in List

The UserID relates to the ID of the user within the site collection. This is fine but the chances are that not all users are members of the site collection. To address this, the SharePoint OM has a handy little method called SPWeb.EnsureUser() which accepts a Login Name of a user as a string and first checks if the user exists in the site collection, if it does not then it adds the user and returns an SPUser object. Please be aware that you may need to set SPWeb.AllowUnsafeUpdates = True for this to work.
From the SPUser object it is very simple to get the ID (SPUser.ID.ToString() and SPUser.LoginName.ToString()) and then use simple string building to get the string in the right format for the ‘Person or Group’ field.

string sAllContacts = "";

using (SPSite site = new SPSite(“http://sites/site/yoursite”))
{
    site.AllowUnsafeUpdates = true;
    using (SPWeb web = site.RootWeb)
    {
        web.AllowUnsafeUpdates = true;
        string[] aAliases = sAliases.Split(';');
        foreach (string sAlias in aAliases)
        {
            SPUser user = web.EnsureUser(sAlias);
            sAllContacts += user.ID.ToString() + ";#" + user.LoginName.ToString() + ";#";
        }
        web.Update();
    }
}