First you will need the ID of the pipe you want to add members.
 

Then just run the following mutation:
 

mutation { n1: inviteMembers(input: {pipe_id: pipe ID, emails:  { email: "email@email.com", role_name: "permission level"}}){    clientMutationId n2: inviteMembers(input: {pipe_id: pipe ID, emails:  { email: "email@email.com", role_name: "permission level"}}){    clientMutationId n3: inviteMembers(input: {pipe_id: pipe ID, emails:  { email: "email@email.com", role_name: "permission level"}}){    clientMutationId  }}

For the permission level use:
- admin for pipe admin;
- member for pipe member;
- read_and_comment for restricted view;
- my_cards_only for read only and
- creator for start form only.