Of course they can sign up, its just a 90 day promtion where each day the admins for the companies on Twitter and the other 190M users will se GO800 advertising, just like you see the 40404 text prompt at the top of every single Twitter page right now.
Heck, for all we know, Twitter and GOIG may have a behind the scenes agreement to join forces, a 90 day promotion is unheard of for a major websites like that.
Twitter wants to catch Facebook in net worth and demand, well GO800 may be just one of Twitter's trys to help do that.