Two permission sets are included in the Salesforce package.
Send2CRM Messaging Administrator
Assign this permission set to:
The API user that the Send2CRM service runs under. For public Site APIs, this will be the default record owner.
Any administrators that require full access to the messaging objects.
Send2CRM Messaging User
Assign this permission set to any users that need to use messaging functionality in Salesforce.
If notifications are configured, they will be sent to these users.