Question: In Avengers: Endgame, we see Doctor Strange and other sorcerers 

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>  open multiple portals at multiple locations across the universe to bring all the help they can get to fight against Thanos' army. 
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When last seen, Black Panther, Falcon, Wanda, etc. were in Wakanda, Wasp was in San Fransisco, Ravagers were on their Ship/Planet somewhere in the universe.

So, my first question is, how were they able to open inter-planet portals? Has it happened before?

and the second, how did Doctor Strange and fellow sorcerers know where to open the portals and look for help?

Answer: 
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> how did they know where to open the portal and look for the help?
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**Strange said there was only one way to win**
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Strange, in knowing this, would know everyone's locations, **IF** the Avengers managed to achieve the single future that would lead them to overcoming Thanos and succeeding. 

If they didn't succeed, then everyone would not be where they should be. So it doesn't matter that Strange doesn't know where they are now, because they have already lost anyways.
One doesn't need time stone to open the portal. as explained in Dr. Strange movie anyone can open a portal to anywhere the can imagine (I don't know how he got the Imagin what it looked like on that side of the war) also he got Wang to get some more people to open the portal and fight the war against Thanos.