Of course you can! Not only can you add a user from your account's Users page but you can also add a new user within your Proposal Outline. Look over in the action panel to the right where it says, "Default Workflow" and if you do not have the appropriate user in your Default Writer/Approver options you can change that if you are have the Admin or Manager role within the system. Just select the blue text/icon that says "Add a User" located near the bottom right-hand side of the screen.
After you click "Add a User" a new window will appear where you will fill in the required fields of First Name, Last Name, Email, and the Role. After you fill in all the required information click "Create User & Send Invite."
The new user will be sent an email inviting them to become a user in your company account. After they register their user account will be ready to go!