A meet groupoid is an algebraic structure that is a groupoid and at the same time a meet semilattice with least element. The cosets of open subgroups of a topological group, together with the empty set, form a meet groupoid in a natural way, given by set multiplication in case the associated subgroups match, and the intersection operation. Meet groupoids, in the equivalent form of coarse groups, were introduced by Tent in a 2018 paper with Kechris and the speaker. In joint work with Schlicht and Tent, we use meet groupoids to show that the isomorphism relation between oligomorphic closed subgroups of Sym(N) is Borel reducible to a Borel equivalence relation with all classes countable.