With Clearstream there's no limit to how many users you can have managing your account. Each User only has access to the account/Subaccounts they've been invited to. As an Owner or Admin in the master account, you can invite a User to multiple subaccounts from within the master account settings. There are 3 different roles that a User can have.
Owner - By default the person who registered for the account will be the Owner, but you can transfer ownership from your Settings. The Owner can do everything and has access to the master account and all Subaccounts. There can only be one Owner.
Admin - Admins can do everything that an Owner can except they only have access to the Account or Subaccount(s) they've been invited to. They also cannot create Subaccounts or edit Subaccount allocations. Admins can invite additional Users, edit Integrations, access billing info, API, and change your monthly plan (if a User in your master account).
Editor - Editors have access to all the core features in your account. An Editor can create and send messages, view incoming messages, and create/edit Keywords.
How to Change a User's Role
Go to Settings.
Click Users in the sub-menu
Find the user whose role you'd like to change and click the 3 dots beside their name > Edit.
Select the role you'd like > Update User.