posting is never going to be "simultaneous", or even "fast". however, NetGroup.post() should reach all members of the group, within a few seconds, no matter how big the group is.
a group of 100 clients on a LAN should totally work in serverless mode.
are you sure you're using NetGroup.post() and not NetGroup.sendToAllNeighbors() to send your messages? sendToAllNeighbors() will only send a message to a peer's neighbors, which are the peers to which it is directly connected. a peer will not be directly connected to every other peer in a 100 member group. on average, in a 100 member group, i'd expect each member to have approximately 27 neighbors (2*log N + 13). some will have a few more, some a few less.
if somehow your wireless LAN was partitioned when the members joined the group, the group could be partitioned into disjoint sets according to the physical partition. however, once the physical partition was healed, the IP multicast neighbor discovery should eventually repair the RTMFP group partition with very high probability.
note that P2P on a wifi LAN actually takes twice as much wireless capacity as you think, since peers aren't directly communicating with each other. for one peer to send a mesage to another peer, the message has to go on the radio to the base station, which then retransmits it to the receiving peer, so the message is sent over the radio twice. and when you're talking about a lot of peers (like 100) doing a P2P operation at nearly the same time, you could run into situations where the peers jam the wifi network. you'll need to do some experiments to see how well your clients and base station handle that kind of scale.
Thank you so much for your speedy feedback! I'll try testing it again with 100 tablets now I know what to look for. Is there anything you would recommend to accomodate the 500 tablets I've been asked to cater for?
EDIT: Only using netgroup.post, not any other method.