If someone shares a Server with you, there are two ways in which you can accept the invitation: via the email you receive or directly in your Plex Web App.
Accept Invitation Via Email
When someone shares a Server with you, an email invitation is automatically sent. When you open the email, it will contain a link that you can click to accept the sharing invitation:
Accept Invitation Via Website
After someone shares a Server with you, you can view any pending invitations online in the Plex Web App.
Open Plex Web App to Access Users
You can access these under Settings > Users in Plex Web App.
Click the Friends menu option on the left to access your Friends.
Related Page: Using Plex Web App
Review Pending Invites
On the Friends tab, you can review pending invites.
For each pending invite, you can choose to either accept () or decline () the invitation.
If you accept, the share will be added to your Friends.