How does my manager give me access to a clients files?

All Client files are stored in Microsoft 365 Groups. Regardless of which option you choose below, all of them do the same thing, which is add you to the Microsoft 365 group. This means if you add someone in Teams you do not need to also add them in SharePoint and Outlook. 

Microsoft Teams: Add Members to an Office 365 Group Site

1. If you're a team owner, go to the team name in the teams list and click More options  > Add member.
   
   

2. Start by typing a name, distribution list, security group, or a Microsoft 365 group. You can also add people outside your org as guests by typing their email addresses.

3. When you're done adding members, select Add. You can make someone a team owner by selecting the down arrow next to Member. A team can have multiple owners. 

4. Select Close. People that you add to a team will receive an email letting them know they are now a member and the team will appear in their teams list.

SharePoint Online: Add Members to an Office 365 Group Site

By default, Office 365 group-associated SharePoint Online site permissions are configured with the associated Office 365 group. E.g., the “HR Members” group is added to members with edit permissions in the SharePoint site. So, adding users to the associated Office 365 group automatically gains permission to access the respective SharePoint Online site. To add members to an Office 365 Group Site by adding them into the group, follow these steps:

1. Login to your SharePoint Online site >> Click on the Members link in the top-right corner. 
   
2. Click on the “Add members” button in the Group membership panel.

   

3. Enter the User name in the Add members page and click on “Save”.
4. Now, you can click on the little drop-down arrow to set the permission for the user – Member or Owner.