After reading and understanding Usergroups and Secondary Usergroups you should follow to this instruction:

1. Then create for example two Secondary Usergroups (SG1 and SG2) for two forums (F1 and F2)
2. Set "No Access" for SG2 and"Standard Access" for SG1 in F1 forum
3. Set "No Access" for SG1 and "Standard Access" for SG2 in F2 forum
4. Install the User Custom Fields addon, navigate to Dashboard > Forums >  Member Fields > Field Manager Tab, edit the Secondary Usergroups field and only enable the SG1 and SG2 usergroups.
5. Then go to Dashboard > Forums >  Member Fields > Registration Form Tab and add the Secondary Usergroup field.

After these steps the registration from will show groups in a dropdown. People select whichever they want to participate and register. So they automatically get correct Secondary Usergroup and correct access to the forums.

Important Notes:

1. If you want to totally hide F1 for SG2 or vice versa you should use the "No Access", if you just want to prevent SG2 users to open a topic or post reply just use the "Read Only" access.
2. Also, the same "No Access" or the "Read Only" access should be given to other non-moderator usergroups, such as "Registered", "Customers", "Guests".

Yes, you can do that. Please attentively read this support topic. Here is explained all steps: 

https://wpforo.com/community/faq/wordpress-user-roles-wpforo-usergroups-and-forum-accesses/

I'm using you forum plugin to create a private community for my client.

It's working good all registered user can access the forum that we can manage it in the Backend.

But I have one request from the client.

"it is possible that people can only register for a Club because currently when I register I have access to all clubs. Ideally we should be able to choose which club we join, and we should just have access to the content of the clubs we are registered in."

*club is a forum category

You can tell me if it possible or not or I need to buy an addon or if it need add some custom script, you can send it for me.

Thanks.