[em-bee]

But how hard it is to just tell a person you vaguely know (like neighbors or people who you know from work or venues) "I feel a bit lonely, wanna do something?".. we always have to have a "better" reason than just that.

i think it helps to phrase it differently. don't say "i am lonely", say, "i work alone and i like to sozialize". or "i am a very social person and i don't get enough interaction at work". to a coworker you could say: " i enjoy talking to you, want to hang out after work" or something like that.

i agree that "i am lonely" potentially comes across as needy, so i would focus on the general positive aspects of socializing.

i should note that i didn't have any opportunity to apply these myself but i was occasionally on the receiving side being invited to join others without any specific reason other than hanging out together. which is actually another approach. look for existing groups and see if you can join them