Can I give permission to people to use or modify my designs?
Yes, but make sure that your design is legally protected before doing so. It is always better to give such a permission in writing. This avoids possible problems in the future.
Do you want to work with licences? You can give one or more parties the permission to use your design in exchange for a certain amount of money. As author you remain owner of the IP rights at all times. Parties without a licence are therefore still committing an infringement.
The use of exclusive licences is also a possibility. In that case, as single party gets your permission under specific circumstances. That company is then the only party allowed to use your design.
Make sure that your licence is always limited in time as well as geographically. How to draw up such a licence can be found here.