List of Tekkit compatible mods:
http://mcportcentral.co.za/wiki/index.php?title=Ports_for_1.2.5
You would need to put the file you download into your server's mod folder.
Then, if someone wanted to join your server, they would have to install the client version of the mod.
EDIT:
Be advised that there might be ID conflicts that you have to manually configure yourself. Then, the people who want to join your server has to have the same config as you.