Once the user invitation created, the invitee can accept or decline it. Also, the initiator of the invitation can cancel it, if it is still in the pending state. The list of operations is available under the cta parameter in UserInvitation object (see an example here).

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Accept invitation

Only pending invitation can be accepted by the user associated with invitee context. If invitation accepted, the mutual connection is created and two users are added to each other’s network. To accept an invitation, acceptUserInvitation mutation should be used. It has one required incoming argument - invitationId. 
  mutation acceptUserInvitation {
    acceptUserInvitation(invitationId: 1) {
      response {
        id
        isAccepted
      }
      errors
    }
  }
If the mutation executed successfully, a UserInvitation object will be returned in the response.

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Decline invitation

Pending invitation can be declined by the user associated with the invitee context. If invitation declined, the connection between two users will not be created, and the new invitation can be sent to the same invitee. To decline an invitation, use declineUserInvitation mutation. As in the previous example, this mutation has one required incoming argument - invitationId. 
  mutation declineUserInvitation {
    declineUserInvitation(invitationId: 2) {
      response
      errors
    }
  }
If mutation executed without any errors, boolean value will be returned in the response indicating the execution status. If true returned, mutation has been executed without any issues, and the invitation has been declined.

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Cancel invitation

Pending invitation can be declined by the user associated with the initiator context. The action is similar to the invitation declining. To decline an invitation, use cancelUserInvitation mutation. This mutation also has one required incoming argument - invitationId. 
  mutation cancelUserInvitation {
    cancelUserInvitation(invitationId: 3) {
      response
       errors
    }
  }