How do I store a list of items in a custom data attribute?

With the API or from the UI, I can create custom data attributes. These attributes can have one of string, integer, float, boolean, or date values. However, I need to store a list of values in a field.

For example, a user (contact) can be in many teams at once in our system. But with the above model, I can only have one team at a time.

What are the possible ways to implement the above functionality with the API/SDK? Should I use tags, or something else?

Thanks in advance.