code: public CMASpaceMembership setEmail(String email) {
      if (email == null) {
        throw new IllegalArgumentException("email cannot be null.");
      }
      if (!email.contains("@")) {
        throw new IllegalArgumentException("email needs to contain an '@' symbol.");
      }
  
      this.email = email;
      return this;
    }
docs: Set the email address when creating a new membership.
  
  @param email to be used for inviting people.
  @return this membership for ease of chaining.
  @throws IllegalArgumentException if email is null.
  @throws IllegalArgumentException if email does not contain an '@'.