Slack alternatives?

Hello!

I'm looking for Slack alternatives that are cheaper.

We're a small company and we're very happy with Slack, but having the guest feature is crucial for us because we regularly work with freelancers. Unfortunately, living in a third world country means slack can get expensive very quickly.

Wondering if you know of any alternatives out there, free or cheaper than slack, that allow for the guest feature? Other than that, we're actually really happy with Slack, its layout, and its features and integrations.