trailboss is correct. The pier at Denlamen has a locked gate - uses a 4 digit access code. They told us you had to stay there to use it (we were staying at Den Lamen). They were even kinda cranky about if you are at Den Lamen, don't use the Sand Dollar beach.
There isn't a gate at Buddy's or Captain Don's.
If you want to dive Bari - you can get in to the left (facing the ocean) of Den Lamen, I saw a bunch of folks doing that last time I was there. I cant comment on the entrance, just the fact I saw a lof folks do it and no dumps.
I vaguely remember there being a sign that said something about 5 dollars to dive Captain Don's for non-staying guests - could have been an old relic from the past.. It was on bar gazebo. One point, the dive shop closes early at Captain Don's (5?), a couple associates will be having a beer for a while but I really don't know how you would pay anyone or ask anyone if you came by at sunset.
All that said, I highly recommend doing captain don's or buddy's for your first night dive. It wont be the most amazing sights there, but it is very easy entrance off the docks and easy navigation by following the line back to the docks - they go down at least 60' last time I was there.
I got my wife and daughter comfortable doing night dives there. Once you are comfortable, I would do Bari, we saw a bunch of neat stuff there.
Remember to save some air to hang around the shallows and look, a lot of really cool stuff comes out.
Also, I dive normally with just a natural wetsuit, for night dive, I always put on a full rashguard, just in case.