Is it dangerous for our computers to go into the site since the security certificate has expired?
Yes and no.
It doesn't matter you connect to the site with an outdated certificate, the traffic is still encrypted and FreeMoney just said he's aware of the problem, so the error isn't on your end.
However: Since the certificate isn't valid, an attacker could launch a MITM-attack and you would get a similar error, so there's currently no way of knowing whether you're getting that error because of the current problem or because of an attack.
So I would recommend _not_ logging in as long as the error's not fixed. If you have Bitcoins on that site, and if you fall victim to a MITM-attack now, your Bitcoins will most likely be gone. Just sit tight and wait till it's fixed.