Ready an action.

[Dramakus]

Is it possible to ready an action to attack and gain sneak attack after an ally moves into position, attacks and also gains sneak attack?

[Chess Pwn]

yes

[Helcack]

Yes.

[Claxon]

If I understand your question it is more clearly stated as the following:
1) You are threatening your target with a melee weapon
2) You have an ally who is not threatening the target, but is nearby
3) On your turn, you ready your action to attack when your ally moves into position
4) Your ally moves into position and attacks. Since you both are threatening with melee weapons, the target is flanked. Since the target is flank, you qualify to deal sneak attack damage.
5) Before your ally actually attacks you get your attack, which as noted above qualifies for sneak attack.

Yes, absolutely it works.

[FLite]

What you *cannot* do is:

A. Five foot steps
B. Five foot steps into flank
A. Full attacks
B. Full attacks.

(Have seen GMs do this when multiple rogue mooks were going on the same initiative.)

[Archaeik]

However

Combat:Ready wrote:

You can take a 5-foot step as part of your readied action, but only if you don't otherwise move any distance during the round.

[FLite]

Yes, but since you can't ready a full attack, that still doesn't let you do that.