Hello
How can my friend join to my host if he has the steam version?

whomever hosts the game has control so for example lets say you host a game on Gog [see image]
you have to check that you and everyone else has the same game version i.e, build
you have to click the cross platform option if you want your Steam mates to join
you have to have no mods or the same mods on each player

which ever person hosts the game [you in this example] is the one that Dlc matters for so if you have 5 dlc then every player that joins your game also gets those 5 Dlc... even if they don't own them

thx for the answer thou the problem was something else

we fixed it with:
steam user have to use a cross-platform beta on steam to be able to join