The "Site Permissions" link is where you go to view and manage permissions for the site. If you simply want to add a user to a group that you know has access to the site you can click on the group link in the "Groups" section in the left-hand navigation. If you don't see the group just click the "Groups" link in the left-hand navigation and a list of groups will appear to the right.
When a site is created the default option is to inherit the security settings from the parent site unless otherwise specified. Notice in figure 1 under the title "Permissions: Human Resources" the description, "This Web site inherits permissions from its parent Web site". If you need to modify the site's permissions click the "Action" menu item and click "Edit Permissions". This will copy the permissions from the parent and the site will stop inheriting. At that point you can edit, add or delete permissions.
Add a User
To add a user click the "New" menu item and click the "Add Users" link. You may also create a new group for the site.
Delete a User
To delete a user or group, select the checkbox next to the user or group. Then click "Actions" and click "Remove User Permissions". This option only becomes available after the site stops inheriting permissions from its parent.
Edit User Permissions
To edit a user or group's permissions simply click the user or group link or select the checkbox next to the user or group, click "Actions" and "Edit User Permissions"
To inherit permissions from the parent site click "Actions" and "Inherit Permissions". Be careful, doing so will remove all the custom permissions on the site.