func (s *IssuesService) AddAssignees(ctx context.Context, owner, repo string, number int, assignees []string) (*Issue, *Response, error) {
	users := &struct {
		Assignees []string `json:"assignees,omitempty"`
	}{Assignees: assignees}
	u := fmt.Sprintf("repos/%v/%v/issues/%v/assignees", owner, repo, number)
	req, err := s.client.NewRequest("POST", u, users)
	if err != nil {
		return nil, nil, err
	}
	issue := &Issue{}
	resp, err := s.client.Do(ctx, req, issue)
	return issue, resp, err
}
AddAssignees adds the provided GitHub users as assignees to the issue.

GitHub API docs: https://developer.github.com/v3/issues/assignees/#add-assignees-to-an-issue
package github
import (
	"context"
	"fmt"
)
func (s *IssuesService) ListAssignees(ctx context.Context, owner, repo string, opt *ListOptions) ([]*User, *Response, error) {
	u := fmt.Sprintf("repos/%v/%v/assignees", owner, repo)
	u, err := addOptions(u, opt)
	if err != nil {
		return nil, nil, err
	}
	req, err := s.client.NewRequest("GET", u, nil)
	if err != nil {
		return nil, nil, err
	}
	var assignees []*User
	resp, err := s.client.Do(ctx, req, &assignees)
	if err != nil {
		return nil, resp, err
	}
	return assignees, resp, nil
}
func (s *IssuesService) IsAssignee(ctx context.Context, owner, repo, user string) (bool, *Response, error) {
	u := fmt.Sprintf("repos/%v/%v/assignees/%v", owner, repo, user)
	req, err := s.client.NewRequest("GET", u, nil)
	if err != nil {
		return false, nil, err
	}
	resp, err := s.client.Do(ctx, req, nil)
	assignee, err := parseBoolResponse(err)
	return assignee, resp, err
}