Depends on what you want from a team and what kind of paintball you are looking to play
For tournament play you need the right size team etc
However it is not always necessary to be in an organized team. Probably not so much for main tournaments but scratch teams can be put together for a particular event. Contact sites / organisers expressing an interest and people can be put together
For general speedball style play (I expect) that walkons etc can have people put together on the day
For general woodsball, scenario etc it is not necessary to be part of a team. Certainly in scenario you can go as a team, group or an individual and when playing you can stick to your group, play alone, tag on with a team or group or just get together with whoever you meet each time you respawn
In the scenario community teams are together for a variety of reasons and could be just a group of mates having a laugh, a social group or a more serious team.
You can therefore fairly easy become a team - provided it suits what you want.
To be an organised team takes more effort and involves things such as your team identity which could mean co-ordinating your clothing, badges etc and then someone has to keep the momentum of getting people to pick the events, get booked in (individually or getting money together to book as a group)
Joining a team is a matter of fulfilling the requirements of a team which often involves getting noticed, playing alongside them and then getting invited in / trying out for them
There will be different commitments for being part of a team - turning up for certain events, taking part in running events, marshalling events or your local site, meeting sponsorship obligations etc
If you plan on joining a team you should also consider what you buy. It's not too clever to splash out on all the latest gear for a season - and then joining a sponsored team that can only use a specific companies gear