adding usre to sharepoint group using powershell

Dear All,

I am new to power shell script. I have below question. Please help me.

I have 1000s of site and subsite, i want to add a sharepoint user in all default "Visitors group" of site and subsite, but my group name is looking like "site name Visitors". I need a powershell script to add my user to Visitor group. could you please help me on this.

Thanks in advance

Hari