Yes! So groups and segments are two different things. You can create a group in your audience for contacts to belong to, which are populated by importing a list into a specific group or by allocating them manually. If your signups are created to do so, your customers can also choose which groups to which they would like to belong.
Segments are used to pull contacts based on a certain set of criteria that you determine. Here's some more information on creating segments in case that is helpful!
When sending an email, you can send to both groups and segments, and you are correct, those contacts that do not belong to a group or segment will be "in limbo" and will not receive an email. The best thing to do is create a Limbo segment and assign all of those contacts to a group, that way they will receive your mailings! (Instructions on how to set up a Limbo segment are in the above attached article).
Hopefully these steps help when sending to your audience! Let me know if you have any other questions,