Those who are already members here have good interest in the field of crypto and majority has invested. Now, these people might be in contact of many other people who are also interested in this field and this they can invite such users here.
People come with their own interests and not to make their friend happy. The latter can happen but then they will leave the forum very soon too.
When such users having own crypto related experiences come here, they will share it and it will result in further organic growth and expansion of the forum. The chain will continue.
I dont think the number of such users in many countries are high that one can just walk in their neighborhood and ask "who loves bitcoin" and get a overwhelming "Me, Me, Me" response, rather "its a scam!", "crypto is fake" responses.
1) Organizing a contest of bringing referrals to the forum and giving monetary reward to people with most referrals.
Dont want to sound racist, but there are certain populations that bring in all their family members to make accounts as many as possible just to defraud campaigns. Have you tried looking to the "Reputation" section for alt accounts. Who is going to take the time to verify every such sign up?
2) Another option is to give membership upgrades instead of monetary reward.
Any type of incentivization should be avoided. This forum already gets a lot of traffic and this is not needed.
Can it do any good to the bitcointalk forum?
It might have, if we had an army of workers alloted to scanning every sign up and that too several years back to start with. A lot of forum drama would not be existing then.