Hi, welcome!
If you press the "n" key, you can see the list of names (connection box) of the connected players.
By clicking SS next to their name in the list, you can send your current setup.
They will see a message in the chat history saying "x has sent his/her setup".
When someone has sent their setup the "ss" in the connection box changes to a green "s". Click on it; then you will be promted to name the file so that it can be saved.
The file will be accessible when you go to the garage sceen and select the relevant car, and the select the newly saved setup from the list.
Hope that explanation wasn't too complicated!