There are two methods
Go to your section. In the 
Section Admin panel at the bottom of the navigation

Click on Members



1. Send a Section Join Link 
You can copy the 
Join Link which looks similar to this:
and is found above the members



You can send to anyone you want to join your sectionIf the user is new to the client, they will be sent to a registration page to sign up. Once they complete the registration and authorization, they will be automatically added to the sectionThis is join link self-registration strategy is great for when you have a large number of users to add.

2. Manually add users
On the Members Page, locate the Add User button above the members list


Type users' names in the box. Registered users names will appear as you search them. You can add more than one member at a time using this method. Click Add Members