If you accept the invitation, you’ll immediately gain access to the shared account on your home screen. You don’t need to do anything else to accept the invitation. 

Hot tip: as an invitee, you don’t need to open a shared account yourself. If you were to open a shared account yourself, you would need to close that account before you can accept an invitation to another shared account. So be sure to check with your + 1 beforehand who will be inviting whom.