Hi @MySql Backup & Restore , and welcome!  If you want me to change your user name, just send me a private message).

I believe you are refering to this section:

Select one of the following:

• Media kit - Create a new CommCell Group: Select this option if you plan to install the CommServe, install feature release updates, install additional agents on an existing CommServe computer, or repair an existing instance.

• Media kit - Join an existing CommCell Group: Select this option if you plan to install a new client, install feature release updates, or install additional agents on an existing client.

• Advanced – For package based selection

You should sele3ct the second one.  The instructions are below:

https://documentation.commvault.com/11.25/expert/2009_installing_commvault_locally_on_unix_linux_and_macintosh_computers_using_installation_package.html

@MySql Backup & Restore  

If you are referring to Commvault Client Groups then follow books online link to create the groups 

Link → https://documentation.commvault.com/11.24/expert/4876_creating_client_computer_group.html

If you are referring the the Unix group then connect to mysql server as root and run id <mysql_user> 

This will show the unix group which is part of the mysql user used for the backup.

Provide the Unix group name during the installation.

If you do not have this information while installing the agent the continue the installation without providing the Unix Group, however once the agent is installed connect to the Unix/Linux server and use cvpkgchg command to reset the group on the commvault install

Link → https://documentation.commvault.com/11.25/expert/1935_changing_unix_groups_and_permissions_on_unix_clients.html