You can choose whether to participate both in Network Opportunities and in Custom Opportunities.
To see Custom Opportunities, you will have to go to Network --> Opportunities --> My Opportunities.
New opportunities in the listing display a status that indicates that you have not participated in that opportunity yet.
To participate, hover over the opportunity and click on the participate icon. You can also choose to ignore the opportunity if you are not interested. Keep in mind that you will not be able to participate in ignored opportunities afterwards.
To see Network Opportunities, go to Network --> Explore --> Opportunities.
If there is an opportunity that you would like to participate in, click See Opportunity to read its full description.
When you click participate, you will be able to send a message and a quote to the requester.