cloud-manager | VM cluster manger on vSphere provider | User Interface library

So, question is what is the Group that the agent is referring to. Where is that configured? Or how do I find out which Group the agent refers to and how do I check if the group is configured correctly.

Configuration values for the Cloud Manager agent (such as mmsGroupId and mmsApiKey) are set in the config file, which is /etc/mongodb-mms/monitoring-agent.config by default. The agent needs this information in order to communicate with the Cloud Manager servers.

For more details, see Install or Update the Monitoring Agent and Monitoring Agent Configuration in the Cloud Manager documentation.

It kind of looks that the cloud manager now needs to be configured with the seed host. Then it starts to discover all the other nodes in the replicaset.

Unless a MongoDB process is already managed by Cloud Manager automation, I believe it has always been the case that you need to add an existing MongoDB process to monitoring to start the process of initial topology discovery. Once a deployment is monitored, any changes in deployment membership should automatically be discovered by the Cloud Manager agent.

Production employments should have authentication and access control enabled, so in addition to adding a seed hostname and port via the Cloud Manager UI you usually need to provide appropriate credentials.

I have a problem trying exec'ing into a container

As you see, Kubernetes is trying to connect to your nodes use the names like worker1, which cannot be resolved in your network.

You have 2 ways to fix it:

1. Use real FQDN for all your nodes which can be resolved. Usually, VMs in clouds have resolvable DNS names, but it looks like in DO they are not. So, you need to create domain names manually and point them to your servers. Try to check DO DNS service. I recommend you that way, it is always good when you have your own DNS names for your infrastructure.
2. Make your names like worker1 resolvable for Kubernetes components, maybe some custom DNS server or records in /etc/hosts.

UPD:

From @Richard87, for future reference: the third way is to use option --kubelet-preferred-address-types=InternalIP,ExternalIP,Hostname.