Anyone can participate in a Member Group, although as explained in more detail elsewhere, only Institute members can vote at meetings. The mechanisms for joining a Member Group include using the web site www.bcs.org or contacting Customer Services.
A Member Group can apply conditions of entry to the group, e.g. a group for young professionals might have an age limit. In general, conditions should be seen as reasonable and necessary for the group to be credible (e.g. BCS Women or ELITE) without discouraging those who wish to learn from those with more knowledge and experience.