First of all, do multiple copies of it stack? [e.i. If I have two on the field, will my opponent have to pay 4 mana for each attacking creature (2 mana for each sphere, per creature)?]
Secondly, if an opponent has a creature like Geist of Saint Traft which makes a token when it attacks, does that opponent have to pay mana for that attacking token?
Yes they do as as demonstrated by Propaganda and card:Norn's Annex working in tandem and why propoganda like effects are so useful.
Yes, each Sphere of Safety has it's own ability which triggers separately, and since they're both enchantments, they each "buff" each other. With 1 out, your opponent has to pay 1 for each creature attacking (assuming no other enchantments). With 2 out, it's 4 mana for each creature (2 mana 2 times). With 3, it's 9 for each creature (3 mana 3 times).
Abilities that trigger when a creature attacks only happen when declaring an attacker. A creature that's put onto the battlefield attacking isn't declared as an attacker, so the Angel token wouldn't have to be paid for.
Yes, each Sphere of Resistance imposes an additional cost for a creature to attack, and additional costs from any number of sources stack.
Regarding your second question, to "attack" with a creature means to declare it as an attacker at the beginning of the declare attackers step. The angel token created by Geist of Saint Traft is just put directly onto the battlefield attacking, so it isn't subject to the "tax" from Sphere of Resistance .
@hunter9000: Sphere of Safety doesn't have any triggered abilities. Triggered abilities always start with "when", "whenever", or "at". Sphere has a static ability that creates an additional cost to declare a creature as an attacker.
Yes, multiple copies will stack, since they have to pay for each copy of Sphere of Safety separately. Second, they won't have to pay mana for the attack token as it is never "declared" attacking, but is put directly onto the battlefield attacking. This also applies to Jace, Architect of Thought 's +1, and the token won't get the -1/-0 for attacking, and Gideon Jura 's +2, as the token can be set attacking the player directly and not Gideon himself should he want to..