Assigning a new Owner to my Island

Hello. I am quite new to SL. My institute has leased some land in SL. Being the researcher I am assigned as Owner in the contract and now in the Island. However, i wish my co-researchers to have the ability to build and alter our Island. To do this I understand (rightly or wrongly) that they need to be Owners. I assigned as members but they cannot alter landscape (flatten hills for example). In my Group that my co-researchers belong to there is no option to assign as Owners. My 2 questiosn are:
1. How to assign co-researchers as Owners?
2. What other rights or level can I assign if I want them to build in my Island.
Thank you in advance.

The problem is that the group needs to own the land.

You can do this in the about land window. Use the options in the first tab to deed your land to the group, and not just "set" it to the group.

Once you do that the powers you gave them will take effect. They do not need to be owners.

Thank you very much