When you need to deploy software or simply work together with someone from far away you usually have several choices on how to accomplish it. The simple solution is to use something like TeamViewer but if all you need to share is a shell, there an easier way.

using screen it is possible to share a shell session across several SSH sessions. The first thing to do is install screen:

aptitude install screen

Now you have to decide which user should be the host for the shared sessions, i.e. which privileges are required during the session. I would recommend to use an unprivileged user.

Next, ask all your external partners to generate a SSH key pair and send your their public key (preferably in an encrypted email).

For each user who will participate in the shared session, create a separate user