As soon as your integration accesses Zendesk, it will find all agents and groups. G to Integrated Processes > Integrations and in the particular Zendesk integration, click on "manage".
You will first see the list of agents available agents and their Zendesk roll:
If you want to have all agents available in babelforce for routing calls and handling other processes, then just click on "Synchronise All". It will then add all the agents to the babelforce Agent set.
If any of the agents you try to add clashes with an existing agent that you created in babelforce, you will get a little warning, e.g. that the phone number or the email of the specific agent is not unique. These clashing agents will then not be synchronised. You can then decide to fix the clash either by changing the agent already in babelforce or the agent in Zendesk.
If you just want to synchronise some of the agents, then click on the little sync symbol beside each agent.
Take a look at Groups and you will see that it works in much the same way. Note that just the group names will be synchronised. You will most likely need to use Groups a bit differently in babelforce to organise your call distribution approach.