If a GCP reseller wants to monitor existing GCP projects of a client that are associated to a client billing account ( meaning the client is directly paying to Google for it ) , how can the reseller start monitoring those ? What are the activities around IAM that enable the reseller to start monitoring the client projects?
Using the Cloud IAM(Identity and Access Management), any member can be assigned with the access/control over the GCP resources/projects. You can check this documentation for various roles that can be given to a member.
For your specific concern over 'monitoring the billing', a member (in your case a MSP provider) can be given any specific role under “Cloud Billing Roles”. It will help that member to view/manage various billing scenarios over the GCP project/resources.
Apart from your use case scenario, you can also check this blog for various updates over MSP partners. You can make valuable suggestion by commenting there as well.