Meteor upsert - using addToSet and each - mongodb

Question

I have a Groups collection that contains an eventId and an array of guest ids. I am trying to use upsert to check if an eventId and current guest id already exist, and if so update the collection with the new guest id pushed to the guests array, otherwise insert the new collection with both the current guest and new guest. Here's my upsert code:
var groupId = Groups.upsert({
$and:[
{eventId:groupAttributes.eventId}, // find all the Groups associated w/ this event
{ "guests": {$in: [guest]}}, // and include this guest
]
},{$set: group,
$addToSet: {guests: {$each: [ guest, groupAttributes.guest ]} }
});
What actually happens is a new Group is created everytime, and it's because the $each modifier isn't allowing $addToSet to add multiple values to an array as described here http://docs.mongodb.org/manual/reference/operator/update/addToSet/#up._S_addToSet
Instead what happens is the guests array is added with an "$each" array of size 2.
[{"$each":["yQZfEXfs7J9E4Nbqf","s2rk5KAxq4dYtDFNG"]}]
Two questions. 1) Is there a better way to do what I am trying to do, and if not 2) What am I doing incorrectly? Thanks.
Edit: Trying the following code in meteor mongo
db.groups.update({
$and:[
{eventId:"wGMhaP7t4nsiTNHt5"},
{ "guests": {$in: ["yQZfEXfs7J9E4Nbqf"]}},
]
},{$set: { userId: 'gBuR448nsJMcpwjsT',
author: 'Martin W',
weddingId: 'rz9xjtDm3bFAeiCxM',
eventId: 'wGMhaP7t4nsiTNHt5' },
$addToSet: {guests: {$each: [ "yQZfEXfs7J9E4Nbqf","s2rk5KAxq4dYtDFNG" ]} }
}, {upsert: true})
The result is the following
{ "_id" : ObjectId("528fa31a098141f336688d96"), "author" : "Martin W", "eventId"
: "wGMhaP7t4nsiTNHt5", "guests" : [ "yQZfEXfs7J9E4Nbqf", "s2rk5KAxq4dYtDFNG"
], "userId" : "gBuR448nsJMcpwjsT", "weddingId" : "rz9xjtDm3bFAeiCxM" }
Now I try the same code in Meteor
var groupId = Groups.update({
$and:[
{eventId:"wGMhaP7t4nsiTNHt5"},
{ "guests": {$in: ["yQZfEXfs7J9E4Nbqf"]}},
]
},{$set: { userId: 'gBuR448nsJMcpwjsT',
author: 'Martin W',
weddingId: 'rz9xjtDm3bFAeiCxM',
eventId: 'wGMhaP7t4nsiTNHt5' },
$addToSet: {guests: {$each: [ "yQZfEXfs7J9E4Nbqf","s2rk5KAxq4dYtDFNG" ]} }
}, {upsert: true})
And the result is different (and undesirable)
{ "_id" : "4XgaF6pBGEohQR6pa", "userId" : "gBuR448nsJMcpwjsT", "author" : "Marti
n W", "weddingId" : "rz9xjtDm3bFAeiCxM", "eventId" : "wGMhaP7t4nsiTNHt5", "guest
s" : [ { "$each" : [ "yQZfEXfs7J9E4Nbqf", "s2rk5KAxq4dYtDFNG" ] } ] }
I am a mongo newb so am not sure what to make of it, but I notice how the _id is an ObjectId in the first result and not in the second. And the guests array is the way I would expect using meteor mongo, but using $each as a value in the second result set. Any ideas?