All Groups, including their members are automatically copied to the new platform - no memberships are lost.
[+]Will I continue to receive notifications from my Groups?
Yes, notification subscriptions for Group memberships are kept, but will be sent on an immediate basis. You will however receive fewer notifications, as these are now sent differently to better target relevant members (more on Notification Triggers).
If you were following a Group (without being a member) or following a particular content within a Group, you will temporarily no longer receive these notifications. Group and Content Following will be coming soon! (more on Notification subscriptions)