The easiest way to do this is with a bundle plan!
Here's an example: I want the members of my Mighty Network, Modern Creator, access to the entire Collection of Important Spaces, shown below.
First you will want to create an Internal Bundle Plan that provides access to all Spaces within the Collection. To create the plan, go to Network Settings > Payments > Plans. You will want to be sure that access to the Network is Excluded in the Plan (see below).
On the payment details section, you will want to select the plan to be free. Of course, if you want to charge for access to a Collection, you can also set a one-time or subscription fee for the plan.
Now, all you need to do is send the Plan share link to your members to get them access to all the Spaces in the Collection!
You can also hide the Collection so that only the people you share the link with will be able to see them.
If you choose to keep the Collection visible, you can share a direct link to a Collection by selecting the three-dot menu > Share.
The link will land members at the view of all the Spaces in the Collection. If you added all the Spaces in the Collection to a Visible Plan, members need only click on Space > Access, to pay or gain free access to the whole Collection.