Yes, you can develop together with a teammate. The only restriction at the moment is that only can person can do okteto up to a specific service at the same time. But, for example, you could do okteto up api while your mate does okteto up frontend without any issues.
For your teammate to connect to your private k8s cluster, there are a couple of things that you can do:
You can give them direct access to your cluster. This doc will point you to a few different options.
You can install Okteto Self-Hosted on your cluster (is free for teams of up to 3 people) so they can get self-service access to the cluster and many other benefits.