The best way to set this up is as follows:
1. Create the first user (the parent account or manager).
2. Create the second user, and set "Managed By" to the user created in 1 above.
3. Repeat step 2 for any other users within their organization.
Now, when the user created in Step 1 signs in, they'll be able to manage all the other users you created below them in the hierarchy.
We don't ever recommend having multiple people logging into the same user account as, if one logs out for example, this will log the other users out and they may be in the middle of creating a post or similar.
Ideally every person that needs access should have their own user account and login as this is how the platform is intended to be used.