How to set vector type field as a secondary index in smart contract?

Question

I want to set a group vector as a secondary index in the user smart contract. group is the list of groups. Following is the changes I am doing:

struct [[eosio::table]] usertable {
                        name key;
                        std::string username;
                        std::string contact;
                        std::vector<name> group;

                        uint64_t primary_key() const { return key.value;}
                        uint64_t get_secondary_1() const { return group[value];}
                };

                typedef eosio::multi_index<"users"_n, usertable,indexed_by<"bygroup"_n, const_mem_fun<usertable, std::vector<name>, &usertable::get_secondary_1>>> user_index;

Its giving error

error: value of type 'uint64_t (user::usertable::)() const' is not implicitly convertible to 'std::__1::vector > (user::usertable::)() const' typedef eosio::multi_index<"users"_n, usertable,indexed_by<"bygroup"_n, const_mem_fun, &usertable::get_secondary_1>>> user_index;

I am not getting exactly how to give a vector of name type list as a secondary index. Please help and guide.