[michaelkeenan]

Depending on the context[1] (i.e. a loud, packed bar is ok), a light-but-firm, non-creepy hand on the shoulder of someone in a tightly packed group can alert them that there is someone behind them who should be in the conversation[2]. When they turn towards you, you need to be smiling and have open body language[3], and greet them with a loud, confident voice[4]. They aren't trying to exclude you; they just probably haven't noticed you, or don't know how to talk to you if they have.

[1] On context: my impression is that the louder the atmosphere, the more appropriate/necessary it is to use body language and body contact to communicate.

[2] Have you ever been in the way of a bouncer or glass-collector in a nightclub? They have a certain practiced, assertive way of getting your attention - usually a hand on the shoulder that isn't pushing you out of the way so much as alerting you that someone's trying to get by. It's like that. (Some bouncers and staff are less polite, of course.)

[3] Open body language is mostly about not having your arms between you and the other. If you're holding a drink, don't hold it in front of you defensively; hold it to your side, and maybe lower than you might ordinarily hold it.

[4] If you're like many introverts, your voice is quieter than you think it is, so you need to overcompensate for that.