Give 20 users full access to one mailbox.
To achieve your goal, you can follow this steps:
1) Create a security group by one who is the member of the 20 users. You can follow the below steps:
a. Select My Organization > Users & Groups > Distribution Groups > New.
b. Specify a display name, alias, and e-mail address. In this
example, we'll use Printing Services Staff, corpprintDG, and
c. Select the Make this group a security group check box.
d. In the Ownership section, click Add to add an owner, if necessary.
e. In the Membership section, click Add.
f. In the Select Members page, select the other 19 users you want to add. When you are finished, click OK.
g. On the New Group page, click Save.
Note After you create a security group, the membership is closed.
When membership is closed, only group owners can add members to the
security group, or owners have to approve requests to join the group.
Additionally, only group owners can remove members from the security
2) Assign the security group the FullAccess permission to access the mailbox.
You can run the following command:
Add-MailboxPermission "mailbox" -User corpprintDG -AccessRights FullAccess