How can we add/invite a Guest Member in our Company/Organization

Good day!

We're using the Free Plan of RingCentral Glip. We are having problems with adding a guest as part of our company/organization. We would like to add them as a company member even though they're not using our company domain on their email.

We would also want them to be the admin of our company. Is that possible?

Hope someone can help us or give tips on how to set it up.

Thank you!