menu link in sidebar only when group page is shown

i'd like to implement a kind of group calendar. if i am on my profile page i'd like to see all my groups i am member of with my Events, Meetings, ...
if i go to a Groups page i'd like to see a calendar with all the members and their Meetings.
Do i have to split this in two apps? How can a menu entry only be shown on a groups page (e.g. "Group calendar" . On a members Profile page a menu entry ("Group calendar" will not make any sense cause no Group is selected.
Do i have to develop an app under "...../apps/group/..." instead of "...../apps/user/ ....." as mentioned in the docs?

Thanks in advance