There are three different roles that a user can have: an owner (only one), admin, or editor. Each user only has access to the account/subaccount(s) they've been invited to.
Owner: By default, the person who registered for the account will be the sole owner, but you can transfer ownership to another user from your settings or add admins who share the same permissions. The owner has access to all features and can manage the main account and all subaccounts. They can also assign users to a subaccount (or multiple subaccounts), allocate credits to subaccounts, add users, and change users' roles.
Main-Account Admin: Admins added to your main account have additional permissions that subaccount admins don't. They can access billing, allocate monthly credits to other subaccounts, change your monthly plan, and manage subaccounts — basically all the things an account owner can do, other than closing out the account.
Subaccount Admin: A subaccount admin can do everything that an owner can within the specific account or subaccount(s) they've been invited to. They can invite additional users, import subscribers, and edit integrations and APIs within that account. They cannot access billing or manage other subaccounts.
Editor: Editors have access to all the core features in your account. An editor can create and send messages, view incoming messages, create/edit keywords, and manage the inbox. However, they cannot import subscribers, invite other users, access billing, or edit integrations.
There's no limit to how many users you can have managing your account(s), so feel free to add as many of your team members as you'd like!