Group module as possible OG alternative
As per your (3rd) comment like "I personally think OG is ..." below the previous answer: be aware that the Group module is a possible alternative for Organic groups for Drupal 7 these days (and soon Drupal 8 also ...).
Refer to my answer to "How to create basic Organic Groups functionality with flags and references?" for way more details about the Group module.
Specific to this question here, some more details about the Group module, as mentioned on "Group - an alternative to Organic Groups":
Group permissions are defined by the site builder and are then passed onto every group instance. This is done by configuring the “group type”, which is comparable to a node type for nodes. So you can configure a set of permissions for different types of groups, yet don’t have to worry about group members being able to fiddle with them.
In order to make the above work, the Group module defines a concept called group roles. These behave exactly like user roles would globally, but can be defined on the group type level. Group provides three special roles to make your life easier too: Anonymous (no account on the site), Outsider (account on the site but not a member of the group) and Member (account on the site and a member of the group).
So with the above in mind, if you're willing to consider the Group module as a possible alternative, restricting the search results as you are trying to do seems much easier (if not automatically).
Search configuration module for more configuration options
Another approach (which you might possibly want to combine with using the Group module) is to use the Search configuration module. Some details from its project page:
This module has five main functions.
- Alter the appearance of the core node search form.
- Group content types for more meaningful searching.
- Restrict search results by the content type. This is a role based restriction.
- Restrict search results from showing individual items.
- Alter the pager limit (aka number search item results per page).
Its project page contains many more details about what you can do with this module.