add

        # ThreadGroup.add

(from ruby core)
---
    thgrp.add(thread)   -> thgrp

---

Adds the given `thread` to this group, removing it from any other group
to which it may have previously been a member.

    puts "Initial group is #{ThreadGroup::Default.list}"
    tg = ThreadGroup.new
    t1 = Thread.new { sleep }
    t2 = Thread.new { sleep }
    puts "t1 is #{t1}"
    puts "t2 is #{t2}"
    tg.add(t1)
    puts "Initial group now #{ThreadGroup::Default.list}"
    puts "tg group now #{tg.list}"

This will produce:

    Initial group is #<Thread:0x401bdf4c>
    t1 is #<Thread:0x401b3c90>
    t2 is #<Thread:0x401b3c18>
    Initial group now #<Thread:0x401b3c18>#<Thread:0x401bdf4c>
    tg group now #<Thread:0x401b3c90>