Active bloggers tend also to be active readers. Sometimes this manifests in sidebar lists kept on one blog of other sites that the author appreciates or feels are related. In the case of platforms that emphasize social networking (e.g., LiveJournal), an explicit map of linked users may supplant the ad hoc list.
Either way, authors follow other authors' work and roles in the larger blogging community can be very mixed. If you, as a blog author, want to solicit guest perspectives for your site, one way to start is by exercising your role as an audience member. Comment on blogs that you find interesting, open a dialogue with the authors to determine whether the interest is mutual. It would be conceivable at any point, though you may prefer some amount of introduction first, to invite another author to share a guest opinion with your readers.
Logistically speaking, you have plenty of options for implementing a guest post:
- Have the guest author send you the content directly and publish it yourself with attribution;
- On multi-author platforms, provide an account for the guest author with appropriate permissions;
- If you have developed a very close relationship with the guest author I suppose you could give them your account credentials (though I strongly recommend against it);
- Have the guest author publish the content on their own site with a note about it being directed at your audience, and then publish a link to (or quote of) the guest content on your site.
An interesting quirk of the landscape (webscape?) is that the platforms with the greatest emphasis on social networking may also have the least emphasis on long-form contributions and blog identity. Twitter at one end, with its mechanisms for private messages, user replies, hash tags and retweets, "author" and "audience" identities largely subsumed by the open-broadcast, selective-receipt format; Wordpress at the other, with its focus on content management and site/author identity, and fewer networking features. But even in the case of a traditional-format blog that doesn't enable reader comments, there is usually a way to get in touch with the author. It is perfectly acceptable to invite someone via email to submit a guest opinion for publication on your website. The worst they can say is no.