A friend can come on a visitor's visa; there is no sponsorship required. From many countries this visa can be purchased upon arrival at DIA. Check appropriate immigration web sites. Visa must be paid with a credit card, otherwise you have to buy a non-refundable government e-card and then load it with the value of the visa--effectively a donation to the government although the reason stated is anti-corruption.

I think the visitor's visa is for 2 weeks, renewable for up to 8 weeks in total (with appropriate additional fees) (not sure about these durations--anyone?).

Your friend will need a return ticket and if he/she is of the opposite gender (and you're single) there might be complications. Better to have him/her "stay" with a friend of yours who is of the same gender.