To create a Lync meeting in somebody else's calendar, you have to have been given delegate permissions in Lync - if not, you'll receive a message similar to this:
To get delegate permissions, the calendar owner must assign them, which is done in Lync:
1) Click the arrow beside the cogwheel icon in the Lync client.
2) Click Tools.
3) Click Call Forwarding Settings.
In the next window:
4) Select Call Forwarding
5) Click Edit my delegate members
Add delegates as required:
6) Click the Add button
7) Start typing the person's name to search in the contact list
8) Select the person and click OK
9) Click OK on the warning message