A simple way to enable this is to hard-code links to a few groups or spaces that serve as "lobbies," and use the Social Group widget to show all relatefd groups marked with a given tag. So, the Marketing lobby would surface all of the Marketing-related groups. You can also add the Watch-a-tag widget to surface content related to marketing, regardless of whether they are in any of those groups. There are many variations on this theme,and you can find several conversations about creating lobbies here on the site.
Even if you do facilitate the process for people, they should also know how to take advantage of the serendipitous nature of social to find groups. Here are some of the ways I suggest people find groups:
1. Watch the activity stream. If you see content of interest, follow it and check out the group. If it looks interesting, join it.
2. Browse by tag. Again, find relevant content and follow it to the group.
3. Follow the experts. Search for people you know to be experts in a certain area, or find them using the browse people feature. Go to their profiles and see which groups they belong to.
4. Find related groups. In each of the above steps, take a look at the Related Groups widget. This is a great way to find relevant groups that you didn't know existed.
Hope this helps.
You can also encourage them to Browse Groups, and type in what they are looking for (Sales, Marketing, Finance). This also means that it is important for people creating groups to either include those key words in the tags and/or the names of the groups, or it will make it harder.