I go to view a MacMost video at Patreon and it tells me I must join to see it. But I am already a member.

You are either not logged into Patreon.com, or you are logged in using the wrong account.

It sounds like either you are simply not logged into Patreon, or you are logged in, but using a different account than the one you are supporting MacMost from.
▶ If you aren’t logged in, then Patreon doesn’t know who you are and doesn’t know you are a supporter and should be able to view the content.
▶ If you are logged in, but not using the same account, then Patreon doesn’t know that you should be able to view the content either. It thinks you are someone else.
▶ Make sure you are logged in, and make sure it matches the ID that you use to support MacMost. If not, log out and then back in again using the right account.
▶ Note that it is easy to accidentally create a second account if you log in one time with your email address, and another time with the “Continue With Apple” button. Those are separate login methods and would create two separate accounts.
For more help, go to Patreon Support: I have two accounts.

▶▶▶ Also see this troubleshooting video. ◀◀◀