Inviting members by e-mail addresses is a pain when you have a lot of them. If you know they're already on Projectplace you should be able to search and add members from within the application.
Thanks for your feedback. You can invite people by name if they are members in projects that you are also a member of, they are contacts in the project, or if they are part of the same Enterprise Edition. To do this, click on add member in the Member tool and then click on e-mail addresses. You will get a list where you can choose by the above selection and then pick names from a pick list.