When you add a new Collaborator to your Team you can choose which sites they can access. But things change, and sometimes you'll want to add them into more sites, or remove them from sites. Follow these steps to set their access:

Step 1 - Click Teams in the left-hand menu.

Step 2 - Next to the collaborator click the Manage Site Access icon

Step 3 - Select the sites you want to add access to or deselect the ones you want to remove then click Add or Remove Sites.

Now the Collaborator will have access to the sites you've added them into.