Is Group Replication Available in MySQL Community Edition?

I don't think you can get a MySQL Enterprise edition for v5.1.X version.

You can see the diffrence in performance between Community and Enterprise version here:

http://www.mysql.com/products/enterprise/scalability.html

You can also download Enterprise product guide whitepaper to know more details.

In summary additional features in MySQL Enterprise version are MySQL Thread Pool, MySQL Enterprise audit and a Technical support for your server related issues.

The server core is similar (identical?) on the GA and Standard Edition (SE)/Entreprise Edition (EE). What you're getting with SE/EE is stuff like (from here)

MySQL Partitioning
MySQL Enterprise Backup
MySQL Enterprise Monitor
MySQL Enterprise HA
MySQL Enterprise Scalability
MySQL Enterprise Security
MySQL Enterprise Audit

You also get support and some access to goodies on Oracle's MOS (My Oracle Support, used to be Metalink). You can tell your PHB - "look, if anything goes wrong, at least _you_ won't be blamed!".

The funny thing is that many of these "enterprise" solutions are available from open source contributors in various guises, and I for one am convinced that it's possible to have at least an entreprise standard setup using only Open Source components. With the advent of WebScaleSQL, that situation could move up to carrier grade.

As for pricing - see here, it's basically $2.000 (Standard), $5.000 (Enterprise) or $10.000 (Carrier grade) per year. See the features table for a comparison.