julia> v = ["John", "John", "Jane", "Louise"];

julia> mdv = MetadataArray(v, groups = Dict("John" => "Treatment", "Louise" => "Placebo", "Jane" => "Placebo"))
4-element MetadataVector{String, Vector{String}, NamedTuple{(:groups,), Tuple{Dict{String, String}}}}:
 "John"
 "John"
 "Jane"
 "Louise"

julia> metadata(mdv, :groups)
Dict{String, String} with 3 entries:
  "John"   => "Treatment"
  "Jane"   => "Placebo"
  "Louise" => "Placebo"