If you are the owner of a Sharepoint site, you will be able to manage site membership to add/remove members.
Be aware that when adding a new person to the site, that person will have full access to the folders and files on that site - it is not possible to have different access permissions for different folders and files within the same Sharepoint site.
As a site owner you can manage access either within the Microsoft Teams desktop app or within the Sharepoint site in your internet browser:
Click the Teams tab on the left
Find the relevant team you need to manage
Click the 3 dots next to the name then click Add Member
In the name field, start typing the name of the person you wish to add then click the add button
If you want to edit an existing member to an owner or remove a person’s access completely, choose the Manage team option and edit as required.
Sharepoint Browser Site
Open the Sharepoint site in your internet browser
In the top right, click the members button
Click Add Members
Type the persons name to be added and select them and click Save
To edit existing members of the team, click the Members button again in the top right, click the relevant member and edit as required.