Options for you:
If they are free and non-selective, post a thread in the giveaways section.
If they are free and selective, just PM the people you want to give guest posts to. You can't just pick and choose and make a thread about it.
If they are reciprocal and non-selective, that's called "trading". You can't do that until you are a Jr. VIP.
If they are reciprocal and selective, that's called "selling". Can't do that either until you are a Jr. VIP.