Advice and tools for hosting an online journal club
There's an online Astronomy Journal Club hosted on Wordpress.

Here's my main advice: make the process as frictionless as possible. Grad students tend to have very busy lives, and journal clubs will inevitably have high dropout rates unless they have some motivation (even through guilt) to stay in and to continue participating. Keep them updated through some service that they'll constantly check even if they aren't doing science (emails end up annoying people, but Facebook, Reddit, and Google Plus might work). 

It's also probably easier to convince people to set an online journal club when the journal club is about some specialized subject that only a small number of people at any particular university know about (and if they have a strong urge to talk about the subject with people from other universities).

Finally, if there's an academic conference for grad students (for my area, for example, it would be AbGradCon), discuss it with people there! Journal clubs are an excellent way to keep in touch with grad students at other universities.