OOC-activity basically means life. You can do this by thread, IM, or hell you could exchange phone numbers (not a recommendation, a possibility.) The most convenient way is by thread because that way everyone is sure to be able to participate and it shows other potential players the atmosphere in your group and frankly... It gives people something to check on whenever they log in. Rather than just being a handful of individuals, you create a small community where interaction outside of the IC is another attraction to the RP as a whole. 

The reason this is important, and in my experience it scales up in said importance the 'higher' the section, is because IC post intervals become larger. On a big site where there are always replacement RP's, it's very easy for RP's to die. However, taking advanced for example, a lot of people in it have day-jobs or other duties that simply don't allow them to post as often as the majority of the playerbase in the other two sections. In this case you can't keep IC activity up, you simply can't. So it's not uncommon that there's only a post a week. If there is no activity for so long, people begin to detach their selves from the game, because we've all seen it happen before and said RP goes up on the likeliness-to-fluke-o-meter. This is something you generally don't want, so you have to pull them in using another method. The OOC is pretty big on this. It shows that the game is alive and connects players with another, and the more you like your game and co-players, the more fun everyone has.

Furthermore, wherever there is communication, there is misinterpretation. Personally I think it is absolutely essential to put this forward in the OOC or by PM or whatever, rather than just assume something and fly with it IC... Because everybody will get confused, more misunderstandings will stem from it and eventually kinda demotivate everyone.

As for using methods outside of the guild, plenty. However, and this is one of my weaknesses as a GM, I'm far better at interacting with individuals than with groups. I IM with co-players one on one, rather than throwing everyone together. I do regularly talk over steam, though, and as a player in one of Kadeaux' or Brovo's RP's I'm thrown into a steam conference enough, so yeah that counts because it noticeably builds a group atmosphere and brings people together, be it talking about the game or something else. Also. Another thing is using pads for collab's, which really helps out and is technically communication outside of the guild.