Then select add resource.
You can also add roles. Roles can be assigned to resources to help categorize them. Roles could be things like engineer, project manager etc. You can assign a role as a placeholder if you do not know which resource needs to be assigned to a piece of work.
If you have a lot of resources, try the import feature.
Named resources vs resource groups
Named resources are individual people such as "Sarah Smith" and are assigned a role such as "Engineer". Use this type of resource if you need to assign and track work against individual people.
If you do not need to track and and allocate work to individual resources then consider setting up your resources as resource groups. When setting up resource groups, you just specify how many people are in the resource group.
Let us use an example to explain this. Say you have an external company who provide resources for your projects. The individual resources are not important to you, you just know how many people they will have working on the project. You could create these resources as a resource group.