REST API which method / response code to choose?

Question

RESTful API

Endpoint
/projects/{project_id}/users/{user_ids}/join

Example
/projects/1/users/1,2,3,4,5,6,7,8,9,10,/join

Designed to add a new member to the project, re-execution has no actual effect.

Which HTTP method to choose in this case.
What is the response code in case the changes had no effect.

The question arose out of a difference of opinion.

Answer 1

I would do PUT, 201 CREATED to add, 200 OK if no effect