When you see casual play going on between people, especially when it's a group of people, do not assume you can automatically participate. Everything you see—every touch, every grab, every mindfuck, every mode of play—is done by mutual consent. You, the newbie, do not get to assume it's a "free for all" that you can just join in. In fact, you should work under the assumption It Is Not Okay; not until you have permission.
Those people have known each other long enough, have been friends long enough, to play like that. They are well-acquainted, therefore it's okay.
Here's the caveat to that rule: Sometimes you'll know somebody for long enough, and be friends long enough...and it will still be Not Okay.
Not even if that kind of play is obviously okay with them when they're in different company.
Not even if they obviously like playing like that with others.
Not even if you really, really want that kind of play from them.
Being friends, knowing for certain they like to play like that, watching them play like that with other people, even people they don't know as well as you? That does not automatically mean they now have to play like that with you.
Some people think there's this magic clock ticking down somewhere, and once it hits zero, that means they've known someone long enough that they have to agree, or at least submit, to play.
It doesn't work like that.
The interactions, the type and level of play you have with people in the scene, do not depend solely on how long and how well you've known them.
It depends on other things, too.
Personally, I think the biggest factor is chemistry.
There are some people I like to play hard with, because that's the kind of chemistry we have; they make the beast of prey within me rise up to challenge them. There are people I like to play fast and loose with, because they're fun, and they know how to make me feel all soft and yummy inside. And then there are others who I enjoy teasing, because they enjoy the tease, and it becomes a game between us.
And then there are the others I will likely never play with at all, because we just don't have that kind of chemistry.
I still consider them friends. I still share chemistry with them; but it is of a different kind. And the truth is, I shouldn't have to owe them an explanation why I won't play with them.
No means no.
But it's hard to disappoint your friends. They want to play with you, and they don't understand why you're asking someone else to play, and not them; or they're wondering why you respond a certain way to someone else, and not the same way with them. The hard fact is, there may not be a cohesive, articulate reason to give them.
Except, maybe: the chemistry is just not there.
So if you're a newbie standing off to the sidelines, being told "don't assume you can do that too, you haven't known those people long enough," DO NOT take that to mean once a certain measure of time goes by, you'll get permission to go ahead and join in.
In fact, it's a bad idea to make any assumptions at all. Ever.
When in doubt, ask.
And until a safeword has been negotiated, no means no.