Can I have multiple users for the same client log into their account?

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.