How to add a user to your SharePoint site
After you have added your user to the Microsoft Office Portal (MOP), and you have assigned a SharePoint license to that user, you should be able to add the user to the SharePoint site.
On the Site Permissions page, this page will show you if the site is inheriting permissions from its parent or has unique permissions. Since this is the first site under the site collection, this is the main site and will now show you that because its the first site. Here, you can add a user to one of the SharePoint Groups that are already added and has permissions. So, if you want the users to have Read only, just click on the Visitors group and add the user to that group. If you want the user to have edit rights, add them to the members page, and so on. If you want to give a user some other permissions, you can click on the "Grant Permissions" button.
If you choose the Grant Permissions, you will have a window pop up. You can then type the username or email address of the user or group you want to add, then click the "Show Options" link. Once you show the options, you can then assign specific permissions to that user or group. You can then deiced if you want SharePoint to automatically send the user an email by leaving the "Send Email" box checked or uncheck it if you do not want to send an email to the user, and then click Share.
Alternate way to add permissions
Another way you can add permissions to a site is by logging into your sharepoint site, and on the right click Share
Once you click Share, you will have the same options as the screenshot 2 images above. Just type out the users name or group name, click Show Options and choose what rights you want to give the user, and then click Share.