It's easy to invite team members to collaborate with you, and we don't place any team size restrictions on your account. Simply go to your team management page and click the "Invite" button to send invitations to your team members.
To limit which actions a team member can perform, you can assign roles to each team member. We've provided a couple of default roles, but you can create custom ones as well. You can edit the permissions granted by each role (and create new roles) on the role management page.
Our default roles are Account Administrator and Team Member. Both are able to fully manage campaigns and profile settings, but only Account Administrators can access account/payout/billing information and modify team members and roles.