When I loop through a C++ map, why does it give me an instantiation error?

I am trying to loop through a map but nothing I try seems to work... I declare and define the map as follows:

// loop through the strings and group based on length
map<size_t, NGroup> groups;
for (size_t i = 0; i < strs.size(); i++) {
    size_t len = strs[i].size();
    if (groups.count(len) != 0) {
        groups.at(len).appendString(strs[i]);
    } else {
        NGroup g(strs[i]);
        groups[len] = g;
    }
}

with the following relevant classes:

class AnagramGroup {
public:
    map<char, size_t> freqs;
    map<char, size_t> ground;
    vector<string> anagrams;

    AnagramGroup(map<char, size_t> m, string anagram) {
        freqs = m;
        ground = m;
        anagrams.push_back(anagram);
    }
    AnagramGroup(AnagramGroup &other);
};

class NGroup {
public:
    vector<string> strings;
    vector<AnagramGroup> groups;

    NGroup(string str) {
        vector<string> strs = {str};
        strings = strs;
    }
    NGroup(vector<string> strs) {
        strings = strs;
    }
    NGroup(NGroup &other);

    void appendString(string str) {
        strings.push_back(str);
    }
};

And I have tried to loop through the map using the ways listed here:

map<size_t, NGroup>::iterator it;
for (it = groups.begin(); it != groups.end(); it++) {
...
}

for (const auto &p : groups) {
...
}

But I always get the same error:

In file included from prog_joined.cpp:1:
In file included from ./precompiled/headers.h:13:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/cmath:1927:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/specfun.h:45:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_algobase.h:64:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_pair.h:303:17: error: the parameter for this explicitly-defaulted copy constructor is const, but a member or base requires it to be non-const
      constexpr pair(const pair&) = default;
                ^
Line 61: Char 38: note: in instantiation of template class 'std::pair<const unsigned long, NGroup>' requested here
        for (it = groups.begin(); it != groups.end(); it++) {
                                     ^

What am I missing?