"Renewal period" is marked as "(Optional)" when it isn't

This looks like an implementation detail poking through to the UI when it shouldn't. The renewal period is optional in the sense that it isn't required in every case (e.g. teams where membership does not expire), which causes the label to appear.

The form should mark the widget as non-optional for the cases where it requires input.

The 'Renewal period' is optional if you select the 'invite them to apply for
renewal' option as in that case a team admin will be responsible for
actually renewing their membership. For the other options, though, the
renewal is either automatic or performed by the member, so we need a default
renewal period to use.

The only solution I see for this is to use javascript to hide the (Optional)
text when one of the two last options is selected, but I'd like to check
with Matthew if he has any ideas.

I think the underlying problem is by shown by the last sentence of the "Subscription period" caption: "Leave this empty or set to 0 for subscriptions to never expire". Forms shouldn't give people special instructions on how to make a field mean something other than what the label says it means. In this case, there should instead be a checkbox for whether subscriptions expire:

[/] Team subscriptions expire after [____] days

Then all the expiry-related settings should be indented under the checkbox (i.e. their combined left margin should be aligned with the left of the "Team subscriptions expire after" label). And they should be disabled whenever the checkbox is unchecked, instead of being marked "optional".