Implementing Area 51 with Acquia Commons?
Here's a simple solution:


Create a content type, say 'proposal', that any user can add for the petition group (with title as group name and body for reason). 
Users can comment on nodes of that type so they can discuss the proposal.
Create a view to show nodes of type 'propsal' and enable filtering, sorting etc.
Use the Votingapi module and one of the voting widgets (like vote up/down) to allow people to vote on the node.


For the creation of the groups, you can either:


Do it manually, monitoring when a proposal gets a specified number of votes. 
Use the Rules module to do it automatically.