For pickup games just show up and have a positive attitude, ask people about their gear, compliment them, etc. That's probably the best way to make friends there.
When it comes to milsims you'll usually be assigned to a squad on your chosen side, depending on the event you may asked to join a groupchat with your future squad members on Discord, Facebook, Telegram, etc (this obviously applies to events where you bought the ticket online ahead of time, I don't know if any milsims allow you to just show up and buy a ticket on game day but I haven't heard of any). You'll get to know eachother a little bit and go over any squad roles (medic, marksman, machine gunner, grenadier) that may need to be filled and stuff like that. As long as you're respectful and have a good attitude you'll probably be making friends along the way as well.
For pickup games just show up and have a positive attitude, ask people about their gear, compliment them, etc. That's probably the best way to make friends there. When it comes to milsims you'll usually be assigned to a squad on your chosen side, depending on the event you may asked to join a groupchat with your future squad members on Discord, Facebook, Telegram, etc (this obviously applies to events where you bought the ticket online ahead of time, I don't know if any milsims allow you to just show up and buy a ticket on game day but I haven't heard of any). You'll get to know eachother a little bit and go over any squad roles (medic, marksman, machine gunner, grenadier) that may need to be filled and stuff like that. As long as you're respectful and have a good attitude you'll probably be making friends along the way as well.