This tip is for Windows SharePoint Services 2.0/3.0, SharePoint Portal Server 2003, Microsoft Office SharePoint Server 2007, SharePoint Foundation 2010 and SharePoint Server 2010.
The reason I didn’t say in the title of this post “Adding Users to a Site” is that the best practice is not to add user accounts into the SharePoint groups. Rather add Active Directory groups to the SharePoint groups to keep the number of security principals per group down. There is a limit to how many security principals can be added to a SharePoint group. See Plan Site Security for more information.