When adding users into your organization you can set their specific permissions which determines what actions they can or cannot take once they login.
As the Owner you'll be able to invite and remove users at any time so you'll always maintain full control over who can access your account. By setting the user role to Standard you will allow these invited users to manage all aspects of your account with the exception of changing your billing plan. Standard users can add/edit/remove insights and dashboards and will have the ability to create new organizations as well.
By setting a user as Read Only you will be limiting their access to just viewing the dashboards in the organization(s) that they've been added to. Read Only users cannot make any changes to the dashboards, account settings or anything else with the exception of changing their own password and profile name.
If you are an agency who is creating dashboards on behalf of your clients you will likely want to set their user role to Read Only as this is the best way to share the metrics on the dashboard without giving additional account access or visibility. Make sure you create a unique organization for each client you are managing to ensure they can only view the dashboards that you've set up for them.
In the screenshot below you can see how users can be invited to an organization and where their role is set. In addition, as the account owner you can again remove or re-invite users as necessary.