Don't just invite to a group without saying anything
In general, this is a bad idea for anything. You should always send a tell to the person you want to join your group for any reason before inviting.
Do make it a request
Captain Obvious is snickering in the background, but demanding anything from a stranger is not likely to get you what you want.
Do say please
You can even say PLZ if you want, though that may not work with people for whom that is a pet peeve.
Do get your levels right
A level 60 mage cannot port you to Shattrath City, for example. They can't train that Portal Spell until level 65. Mages can port you to Stonard at 35 (if they went there to get the spell) and the major cities in the old world (except Darnassus and Thunder Bluff) at 40. They get the more out of the way city at 50. And if you want to get ported to Dalaran, ask a mage who is level 74 or higher.
Do offer to pay
Portals are not free. They require a reagent that the mage must purchase and keep on hand. Since they cost 20 silver a piece, you should offer at least that much as payment.
Don't make the mage come to you
Go to where the mage is or ask where is convenient to meet.
Don't get belligerent
Even if you do everything right, the mage may not want to port you. Mages don't owe you a port just because they can do it. Just try to find another mage or get to your destination on your own. You certainly aren't going to convince the mage to port you by being angry.
The Spousal Unit later saw Mr. Rudeguywhocan'tspellchief in Orgrimmar about an hour after I refused to port him. I don't know if he found a willing mage or if he made his way from Thrallmar himself. I wish I had asked him if being that rude ever works for him.
Does rudely demanding ports work for any of you?