There are no separate access permissions to activities in Bitrix24. You need to configure access permissions to the CRM entity that the activity is related to.
Go to CRM > Settings > Access permissions > CRM.
Let's configure permissions for the Manager role.
For example, let's configure access permissions to the Sales pipeline in deals. Access permissions to companies, contacts and leads are configured the same way.
By default, all users have full access to CRM. In our example, we allow users to view any activities, but not to create new ones. At the same time, they will be able to add new deals. Grant access to Read action and select All. Restrict access to Update action by selecting Access denied.
In this case, users having the Manager role can view activities related to any deal. However, they can't edit or add new activities.
Access to Delete action works the same way.