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Had the same question myself.
A beta invite does not guarantee a re-invite.
Next beta is likely to come sometime after the holidays.
I found the answer here http://forums.bethsoft.com/topic/1480273-how-often-does-bethesda-send-out-invites/
Originally posted by MMORPGtester Just because you were invited into 1 beta or 2 "those with 2 also participated in the stress test". This does not guarantee you chance to get into other betas tests. You have the same chance as everyone else in the pool.
Actually, you're wrong, but I can't prove you're wrong without referencing the ESO beta forums. So... you'll just have to trust me! lol.
I will say there isn't a guarantee you will get into every new beta event. But it's not completely reset and random invites are sent out as you suggest. If that were true there would not be so many people in all the beta events so far.