This one is pretty straight forward. 

Note: Number of users you can add, depends on your plan. Kindly check Plan Details.

How to Get There?

Settings > Team Members/ Users

To add new user in your account just click on Add button, and you will see

You can choose from three roles

1. Administrator

2. Editor

3. Viewer

What's the difference between these roles?

Editor is the one who can create, edit, update, delete proposals.

Admin can do everything what Editor can do, plus s/he can manage account (settings: add /update team, manage templates, etc)

Viewer: Can view proposals and add comments to those. But s/he can not create new proposals or even edit/delete existing ones.