security implications of participating in app beta testing?
There is no real difference between a beta test application and a regular application for security purposes. You can remove any beta test application the same way you would a regular application. Android is setup so that applications do not normally have access to the data for other applications. 

If a beta test is closed, then the Google+ or Google group is the access control for the beta test. This has no effect on the application on the phone. 

If an application is in the Play Store, you have general assurances that it has been virus scanned. The Play Store is much safer than installing applications from generic websites. The Play Store will not, however, tell you anything about how the application works. 

The permissions requested by the application determine how risky it is to use it. Permissions include things like:


camera
contacts
location coarse/fine
network access
read call log
read contacts
microphone


The general trouble is that applications ask for many permissions, and it is not possible to tell what an application is doing with those permissions. But if an application that turns your flash on and off is requesting access to your microphone or contacts, then you should get suspicious. If an application has access to your contacts and the network, then it is capable of sending all your contacts' information to a server on the Internet.