I would ask first about waiting lists before you look at that.
In our area there is no way you'd get into guides without having been a brownie first. There is a shortage of guide units.
Depending on the time you might or might not get into Brownies. Our area has had times when they've been saying "if you have any friends who would like to..." and times when (like now as a pack's just closed due to lack of leaders) they're not even finding spaces for every Rainbow who wants it.
In my cousin's area for Scouts, if you didn't get your name down before age about 4yo, then you wouldn't get into Beavers, so you wouldn't get into Cubs, and you probably wouldn't get into Scouts.
In our area the waiting list for Scout group things operates strictly on an age thing. So if you joined the list at age 8, you would go straight to the top of the Beaver's list. Which has some common sense in it, although I suspect it wouldn't work so well if there was a huge waiting list.
What I'm saying, is you need to ask at the waiting list places as to what is possible. In our area they've changed from a central list to specific lists in the last couple of years, I believe. So you'd phone up a pack and ask, and they would let you know. Before then, as redsky says for her area, it was a central list.