ometa Membership <: Parser {
  isMember :x   = ?Membership._isMember(x),
  text         = (~'\n' char)+:c           -> c.join(''),
  nonMember    = text:x ~isMember(x)       -> [#isNotMember, x],  
  currMember   = text:x isMember(x)        -> [#isMember, x],
  newMember    = ``add:''text:t {Membership.members[t] = true} -> [#newMember, t],
  line         = newMember|currMember|nonMember,
  linef        = line:l '\n'               -> l
               | line | '\n',
  expr         = linef*:l                  -> l.join('')
}

Membership.members = { }
members = ["Joachim","Dolores","Elvis"]

for (var idx = 0; idx < members.length; idx++){
  Membership.members[members[idx]] = true
}

Membership._isMember = function(k) { return this.members.hasOwnProperty(k) }

group = '
Elvis
John
add:John
Nick
Joachim
John'

Membership.matchAll(group, 'expr') 
[isMember, Elvis][isNotMember, John][newMember, John][isNotMember, Nick][isMember, Joachim][isMember, John]