You know the anti's are only starting on the 14th right? As someone else has pointed out, since Starbucks doesn't want to take sides on this issue and just wants to sell coffee, it might be better to be more subtle than a large group taking over for an hour. Instead perhaps go in ones and twos whenever it is convenient and pay in two dollar bills.
Here is something to think about, most antis don't know this is being planned. Likely most starbucks locations won't know what is going on; if its a large group they will probably think it is some shooting or police club. If its a small trickle all day or over a long time it will look more like what it is: an armed populace.