user group serializer send out stats visibility

[yuenmichelle1]

User Group Serializer to send stats_visibility

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.

[yuenmichelle1]

@zwolf tagged you as reviewer to give you a heads up of the change...but the change is simple enough, i'mma go ahead and merge.