Follow the steps below to add Users to Postal.
Navigate to your Profile by clicking on your name in the top right
Find Profile Settings on the left then select 'Users'
Select 'Invitations'
Invite a new user by filling out their email, assigning them a team, and choosing which role(s) they will receive
Learn more about roles and permission levels here.

Once the Admin invites the User, the User can log in and go between the account(s) that they are granted access to.
Granting Access to Inactive Users

Select your name in the top right and select Users from the Profile Settings section
Select Inactive Users
Search for the inactive user(s) you wish to update
Select "No Access" next to their name in the Roles column
Add the Team and Role
Finally, click Update Roles
Note: if you do not wish your user to be alerted to their access to Postal, then leave the Send User Email box unchecked.

Learn more about adding teams to Postal here.