1) Ask in the support forum for best results.
2) Provide a log so people can see what might be causing the problem (preferably from the host/server as well as the client(s) having the problem).
3) Provide more details about what is happening. For example, is the Invite button disabled in the game? Do they start to join but get stuck at downloading map or somewhere else? Etc.
As it is now, no one will be able to help you as it could be all kinds of causes. You might also want to start by looking at the pinned topic in the support forum that gives you information on doing things like setting up routers and firewalls and antivirus properly for the game.