Skype for Business and Skype

I just started using Skype for Business.  I tried to host a meeting with other colleagues that were just using Skype.  However, while I can find them in the Skype directory, I cannot connect with them ... they are just listed as "presence unknown".  I've tried going from Skype to Skype for Business, and no luck that way either.  Both versions are updated, but there does not appear to be a way to communicate between the two.

I'd say that's pretty worthless.

Any help would be useful.

Thanks.