If you'd like to add someone to your channels and groups for free, you can whitelist him and then send him an invite link.
Whitelisted users will never be kicked from your members-only channels and groups (even if they don't have active payments or subscriptions).
To whitelist a user, open your membership bot and send /whitelist XXX, where XXX is Telegram userId. If you don't know user's id, you can find it in users list (Project > Telegram Bot > List users).
To remove that user from the white list, simply use /unwhitelist XXX.