As an administrator of your organization, you can add members in one of two ways: 

1. By email
2. Sharing an Invite Link

See the below video for a walkthrough, along with steps for inviting members.

Invite by Email

1. Navigate to the Members tab
2. Select Invite Members
3. Enter or paste the emails into the Enter emails area
4. Click on the Send Invites option

Invite with a Sharing Link

1. Navigate to the Members tab
2. Select Invite Members
3. Click on the Create an Invite Link option
4. Enter a registered domain and Tab or Enter on your keyboard
5. Click on the Create Invite Link option and Copy