How to increment filed value of group type node when someone joins the group

So, I am desperately missing OG statistics module for 7.2. There seem to be a way to recreate count functionality it with some Rules help.

I need to build simple counter, which increases +1 when there is new member in the group

What I do:

1. Create "field_user_counter" on group type node
2. Clone rule "OG member subscribe (Active)" which comes included with OG
3. On new rule "Actions"...

I'm stuck on this point.
Any ideas?