Ok, so the first step is to either find a group which matches the type of activity or has an open remit to the type of activities catered for.  Or you set up your own group. 

If it’s your own group you would use the ‘create an event’ option.  Once the event is created, you can either invite people or just see who signs up to it. 

If it is a group you are a member of, you may be able to do the same as above, but depending on the group permissions you may not.  In which case you would contact the group owner, or create a blog entry in the group to request the event be added.