int create_user(const char *name, User **user_ptr_add) {
    // check if name cannot fit array
    if (strlen(name) > MAX_NAME - 1) {
        return 2;
    }
    // allocate space for new user and make new user
    User *new_user = malloc(sizeof(User));
    strcpy(new_user->name, name);
    new_user->next = NULL;
    User *curr = *user_ptr_add;
    // if there are no users, make new_user the first of linked list
    if (curr == NULL) {
        *user_ptr_add = new_user;
    } 
    // else, traverse until the end of the list and add new_user to the end
    else {
        while (curr->next != NULL) {
            if (strcmp(curr->name, name) == 0) {
                return 1;
            } else {
                curr = curr->next;
            }
        } curr->next = new_user;
    }
    return 0;
}

return 1 if a user by this name already exists in this list