Members can join your organization in a couple of different ways:
You can invite members to your organization by inviting them using their email address. This will send them instructions with how to create an online account which can also grant them access to the Enterprise Support channel on Discord.
You can also use the Odin DRM enabled build. This lets your members activate Odin from within Unity without creating online accounts or having a license on hand. You'll be able go get an overview of how many machines uses Odin in your organization member settings.
This can be helpful to automatically figure out how many on your team use Odin.
Additionally, you can also directly assign a license to a user.
You can add organization administrators who can manage licenses but do not count toward license requirements unless they actively claim a license for themselves.