First, you need to make sure you have the role of Owner or Manager in your Organization. Team members with the role of Member can't invite new users.
You can invite new users to the team by doing the following:
Click the + button in the left-nav bar and select Invite team members
Click Profile picture icon and then choose Invite team members
To invite a new team member:
1. click the 
button
2. enter their email address into the email field
3. choose whether you want them to be an Organization Manager or Member.
4. customize which social profiles this new team member will have access to and also
which role they will have. This is only available when you select Member. For Manager you can't do this - Manager automatically has an Admin role on all social profiles.
5. click the 
button
Your new team member will receive an email notification containing a unique link that needs to be used in order to accept the team invite. Invitations that have not yet been accepted are listed under Pending invitations.
In the event that your invitation did not reach the recipient's email inbox ( wrong email address, for instance), you can either resend the invitation or copy & paste the unique link and share it with the team member.