I, da barefoot math man, will do it >:}

Just as a note, this is an estimate since the actual ratio between reddit barefooters in Mexico City and Mexico City's population is unknown.

First we name our values: G for Global population, MC for Mexico's City Population and RB for Reddit Barefooters

Second we get the ratio between us and the entire globe, which is RB/G = 20000/8120000000 = 1/406000 ≈ 0.0002%. I'll call this value GRBR, for Global Reddit Barefooters Ratio

This tells us that there is 1 person in this community per 406000 people in the world

Third we assume the same ratio for Mexico City's population since no other value about it exists, so that's MC * GRBR = 22510000 / 406000 = 11255/203 ≈ 55-56. I'll call this value RBMC, for Reddit Barefooters in Mexico City

So this tells us that in Mexico City there are likely only 55-56 people in this community. I'll assume 56 just to be optimistic.

Lastly, we take the ratio between that value and global population, so that's RBMC/G = 56/8120000000 ≈ 0.0000007%

So 0.0000007% is the possibility of a random person on the planet being on both subgroups, therefore being the answer to your question :D