Hi @sarah.g - welcome to the Community!

Technically, profiles can only belong to a Segment by a “definition” you setup in the Segment builder.  You can’t manually add a person into a Segment.  Here’s a good article that explains the difference between a Segment and a List:

• Understanding the Difference Between Segments and Lists

However, there is a rule that you can use for a Profile that belongs to a specific List with the “If someone is in or not in a list” rule.

See here:

In the example above, it’s a simple definition for anyone belonging the List “Newsletter” will then also be in my Segment “in-list_newlsetter.”  You can further filter it by a certain date or add other conditions to better define the Segment.

Hope this helps!