One fairly common thing that can happen is that a team grows, so that its capacity will increase over time. In a typical roadmap we might model this as changes to capacity on a quarterly basis.
Is there a way to represent this in Portfolio? All I've found is the ability to set a fixed capacity, which is not very useful for any planning further than the current quarter.
To increase the capacity of the team this is going to be a manual adjustment to the static teams capacity as one off events while time progresses.
However you can use combinations of virtual users and set absences or start and end dates to denote a timeframe where the availability of a team member will be increased or reduced to impact the capacity in various ways.
For your scenario of increasing the capacity gradually, while there is not a global method to inflate capacity on an interval you can create a virtual team member and adjust the global availability of that user so that they have a future Start date range, and after that period or during that range the virtual user will fill the gap for the existing team members, See following screenshot for exe but refer to docs listed below for full steps to set the availability start date:
Then once you progress to the point that the capacity is accurately being recorded on the virtual user you could manually update the team capacity and remove the virtual user.
Details on these actions can be seen in the following documentation: