I was going some first chest runs on Skellies for some dino items, and realized I didn't know exactly when the raid locks to prevent late comers from entering (I wanted to invite some friends after I cleared they way to it so they're not stuck doing the first part with me). I checked the wiki and it didn't have any info, so I did some testing to find out. I was going to add it to the wiki, but I've forgotten my login credentials (temporarily locked myself out after trying a few that all failed) and am feeling too lazy to make a new alias there, so I'm putting said info here instead and hopefully someone finds it useful and/or makes it available there...
...though I also noticed a lot of wiki entries for raids that have lockout points do not (no longer? I swore it used to have it) have lockout info in them. :/
Anyways, Skellies locks out the moment you activate the rune that opens the first portal.