There are two ways to add team members. You can either invite them by email as shown below or add them through your Slack integration.
Click on Team settings on the top-left menu.
Click on Team members and then on Invite teammates.
On the pop-up that will appear, select the Role, select Email contacts and add their email addresses.
On the Team members page, you can identify the Login Type each user has. Email users can edit their profile settings once they are logged in.
Email users cannot share pages to Slack.