Having the ability to send out group invites to an entire roster in GRA would be a really nice feature, as my guild uses it for storing who is and isn't on the roster.

Additionally, it might also be nice to be able to automatically send out calendar invites to a GRA roster if this is possible. (We don't put progression raids on our guild calendar because we use a set roster, we use private calendar events instead. Having the ability to send calendar invites to everybody with one button would be nice.)

Roster invitation implemented. I'll release it later.

I'll try to add the feature about calendar events. Maybe one week? Since I'm so busy these days, I can't tell the exact time.

I'd like to add my support for this feature. Since Group Calendar 5 is no longer being maintained it would be good to have a replacement that create private group runs for raids and maintains roles and who is confirmed, sat or whatever. As this info is already maintained by GRA it would seem to be a shortened step to implementation, time permitting of course.

Sorry for the very late reply ...
It would be nice if you can draw me a picture showing the Calendar UI.

Well, the UI is a bit hard to show properly as GC5 is not working anymore, but you can find the addon here: https://www.curseforge.com/wow/addons/group-calendar

There are a couple of images, but it shows more than that.

The first screen shows the menu to create an invite and it's details. If the event already exists (i.e created by the in-game event calendar) theh this page can be used to edit it. Whilst the in-gmae calendar only allows you to set a start date, the GC5 one allows you to se duration from a pull down menu with 30 minute increments.

The invite tab then shows the guild roster by ranks with the usual + or - buttons to expand or collapse each rank. Before each rank and each name there is a selection box which you can click to select the player and invite him to the event. If you click the button by the rank it invites/uninvites everybody in that rank.

Under the group tab those who have accepted can be assigned confirmed, standby or out status and assigned to a role (tank, healer, ranged, melee, standby) to allow you an overview of the overall raid.

GC5 has other features for cross guild cooperation, but these are the important ones.

Oh and you can set each event to repeat automatically every week or on specific days.

Hope this helps and of coruse your implementation may be different.