Availability Hack
We have an availability feature in the road map so athletes can share their availability easily with their coach, but in the mean time I've been working with a coach on a work around that can provide a basic version of the feature with the current tools.
Firstly, create an item in the workout library and choose "Availability" as the workout type. You can create as many of these that suit your workflow. e.g Availability - Morning, Availability - Evening etc.
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In the "Extra Details" of the library item set it so that the client can see it in their library so they can drag it on to their calendar.
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Now you can use the library item, or ask your clients to use it to add their availability to their calendar so you can see it on each day.
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Now you or your clients can use the "Copy day" or "copy week" features in calendar mode to build out the availability across a number of weeks.
You could also optionally create a "Template" plan in your plans library that holds many weeks of availability to easily apply to a client calendar all at once. You can also give your client access to that template plan so they can use it themselves.
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Notifications
If you want to get notified each time a client adds availability to their calendar you can use the "When a client adds an item to their schedule of the "Availability" base type and I am assigned as their primary coach" notification setting under the "Client Schedule Changes" heading. This is also a bit of a Hack.
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